Use include(LocalInstallDirs) instead of include(G

Question

We have a C++ library which provides a CMakeList.txt file, and it leads with the following:

cmake_minimum_required(VERSION 2.8.5 FATAL_ERROR)    
project(cryptopp)

set(cryptopp_VERSION_MAJOR 5)
set(cryptopp_VERSION_MINOR 6)
set(cryptopp_VERSION_PATCH 5)

include(GNUInstallDirs)
...

According to the CMake manual at GNUInstallDirs:

Provides install directory variables as defined for GNU software...

...
INCLUDEDIR       - C header files (include)
OLDINCLUDEDIR    - C header files for non-gcc (/usr/include)
...

Our policy is install into /usr/local; and don't install into /usr. Installing the library into /usr on some platforms breaks the library because the compiler treats all headers in /usr/include as C files, and not C++ header files. OpenBSD is one that breaks us, but it handles the library header files as expected when they are installed into /usr/local/include.

I perform a lot of cross-platform testing, and I don't know of any Unix compatible distributions that fail to support software installations into /usr/local. I know we are safe to install into /usr/local when its available.

I visited CMake modules, but I don't see a choice for something other than GNUInstallDirs. How do we specify something similar to include(LocalInstallDirs) instead of include(GNUInstallDirs)?