C compiler identification is unknown despite setti

I'm using the command:

    cmake CC="C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\bin\cl.exe" 
CXX="C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\bin\cl.exe"  -G "Visual Studio 12
 Win64" -DBOOST_ROOT="c:\local\boost_1_56_0" -DBOOST_LIBRARYDIR="c:\local\boost_1_56_0\lib64-
msvc-12.0"  ..

It says:

The C compiler identification is unknown

and

The CXX compiler identification is unknown

How is it possible when I set those variables right there?

EDIT: Error log file:

Compiling the C compiler identification source file "CMakeCCompilerId.c" failed.
Compiler:  
Build flags: 
Id flags: 

The output was:
1
Microsoft (R) Build Engine version 12.0.30723.0
[Microsoft .NET Framework, version 4.0.30319.18444]
Copyright (C) Microsoft Corporation. All rights reserved.

Build started 21-09-2014 20:17:49.
Project "D:\foo\build\CMakeFiles\3.0.2\CompilerIdC\CompilerIdC.vcxproj" on node 1 (default targets).
PrepareForBuild:
  Creating directory "Debug\".
  Creating directory "Debug\CompilerIdC.tlog\".
InitializeBuildStatus:
  Creating "Debug\CompilerIdC.tlog\unsuccessfulbuild" because "AlwaysCreate" was specified.
ClCompile:
  C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\bin\x86_amd64\CL.exe /c /nologo /W0 /WX- /Od /D _MBCS /Gm- /EHsc /RTC1 /MDd /GS /fp:precise /Zc:wchar_t /Zc:forScope /Fo"Debug\\" /Fd"Debug\vc120.pdb" /Gd /TC /errorReport:queue CMakeCCompilerId.c
  CMakeCCompilerId.c
Link:
  C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\bin\x86_amd64\link.exe /ERRORREPORT:QUEUE /OUT:".\CompilerIdC.exe" /INCREMENTAL:NO /NOLOGO kernel32.lib user32.lib gdi32.lib winspool.lib comdlg32.lib advapi32.lib shell32.lib ole32.lib oleaut32.lib uuid.lib odbc32.lib odbccp32.lib /MANIFEST /MANIFESTUAC:"level='asInvoker' uiAccess='false'" /manifest:embed /PDB:".\CompilerIdC.pdb" /SUBSYSTEM:CONSOLE /TLBID:1 /DYNAMICBASE /NXCOMPAT /IMPLIB:".\CompilerIdC.lib" /MACHINE:X64 Debug\CMakeCCompilerId.obj
LINK : fatal error LNK1181: cannot open input file 'gdi32.lib' [D:\foo\build\CMakeFiles\3.0.2\CompilerIdC\CompilerIdC.vcxproj]
Done Building Project "D:\foo\build\CMakeFiles\3.0.2\CompilerIdC\CompilerIdC.vcxproj" (default targets) -- FAILED.

Build FAILED.

"D:\foo\build\CMakeFiles\3.0.2\CompilerIdC\CompilerIdC.vcxproj" (default target) (1) ->
(Link target) -> 
  LINK : fatal error LNK1181: cannot open input file 'gdi32.lib' [D:\foo\build\CMakeFiles\3.0.2\CompilerIdC\CompilerIdC.vcxproj]

    0 Warning(s)
    1 Error(s)

Time Elapsed 00:00:00.47


Compiling the CXX compiler identification source file "CMakeCXXCompilerId.cpp" failed.
Compiler:  
Build flags: 
Id flags: 

The output was:
1
Microsoft (R) Build Engine version 12.0.30723.0
[Microsoft .NET Framework, version 4.0.30319.18444]
Copyright (C) Microsoft Corporation. All rights reserved.

Build started 21-09-2014 20:17:50.
Project "D:\foo\build\CMakeFiles\3.0.2\CompilerIdCXX\CompilerIdCXX.vcxproj" on node 1 (default targets).
PrepareForBuild:
  Creating directory "Debug\".
  Creating directory "Debug\CompilerIdCXX.tlog\".
InitializeBuildStatus:
  Creating "Debug\CompilerIdCXX.tlog\unsuccessfulbuild" because "AlwaysCreate" was specified.
ClCompile:
  C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\bin\x86_amd64\CL.exe /c /nologo /W0 /WX- /Od /D _MBCS /Gm- /EHsc /RTC1 /MDd /GS /fp:precise /Zc:wchar_t /Zc:forScope /Fo"Debug\\" /Fd"Debug\vc120.pdb" /Gd /TP /errorReport:queue CMakeCXXCompilerId.cpp
  CMakeCXXCompilerId.cpp
Link:
  C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\bin\x86_amd64\link.exe /ERRORREPORT:QUEUE /OUT:".\CompilerIdCXX.exe" /INCREMENTAL:NO /NOLOGO kernel32.lib user32.lib gdi32.lib winspool.lib comdlg32.lib advapi32.lib shell32.lib ole32.lib oleaut32.lib uuid.lib odbc32.lib odbccp32.lib /MANIFEST /MANIFESTUAC:"level='asInvoker' uiAccess='false'" /manifest:embed /PDB:".\CompilerIdCXX.pdb" /SUBSYSTEM:CONSOLE /TLBID:1 /DYNAMICBASE /NXCOMPAT /IMPLIB:".\CompilerIdCXX.lib" /MACHINE:X64 Debug\CMakeCXXCompilerId.obj
LINK : fatal error LNK1181: cannot open input file 'gdi32.lib' [D:\foo\build\CMakeFiles\3.0.2\CompilerIdCXX\CompilerIdCXX.vcxproj]
Done Building Project "D:\foo\build\CMakeFiles\3.0.2\CompilerIdCXX\CompilerIdCXX.vcxproj" (default targets) -- FAILED.

Build FAILED.

"D:\foo\build\CMakeFiles\3.0.2\CompilerIdCXX\CompilerIdCXX.vcxproj" (default target) (1) ->
(Link target) -> 
  LINK : fatal error LNK1181: cannot open input file 'gdi32.lib' [D:\foo\build\CMakeFiles\3.0.2\CompilerIdCXX\CompilerIdCXX.vcxproj]

    0 Warning(s)
    1 Error(s)

Time Elapsed 00:00:00.50

The compiler can be selected by using the CC and CXX variables, but by it's much more complicated than using the -G (generator) parameter. For one, the slashes in the path must be Unix style (/) and they don't take effect after the first time CMake configuration is ran unless the cache is cleared.

For a list of available generator options include the --help option

The paths to the Boost library must also use Unix style slashes (/). Many of the commands will convert Windows paths, but when passing in definitions at the command-line you'll want to always use Unix style paths.

Your command would change to the following:

cmake -G "Visual Studio 12 2013 Win64" -DBOOST_ROOT="C:/local/boost_1_56_0" -DBOOST_LIBRARYDIR="C:/local/boost_1_56_0/lib64-msvc-12.0" ..

You'll also want to make sure that your PATH environment variable is setup correctly for Visual Studio and x64. This can be done easily by opening a VS2013 x64 Native Tools Command Prompt using the command below:

cmd /k "%VS120CoMNTOOLS%\..\..\VC\vcvarsall.bat" amd64

In addition, you should make sure you're clearing out the cmake cache before you try. The easiest way to do this is to delete your cmake build folder, but you can also use the CMake-gui to clear the cache.