'openssl/conf.h' file not found error on M

I am working on a C++ project which uses boost asio. Trying to build the libraries that use Asia, I am getting the following error

/usr/local/include/boost/asio/ssl/detail/openssl_types.hpp:19:10: fatal error: 'openssl/conf.h' file not found
#include <openssl/conf.h>

Looking of solutions here & here, I tried

brew install openssl
brew link openssl --force
xcode-select --install

But didn't help.

Doing the following also doesn't seem to work

export C_INCLUDE_PATH=/usr/local/include
export CPLUS_INCLUDE_PATH=/usr/local/include

Boost version I am using is boost_1_63_0. I am on MacOS Sierra with Xcode 8.3.1. I have installed boost using Homebrew

brew install boost

As I understand from other links, Xcode is looking at the wrong place for ssl headers. But how can I resolve this?

I looked into my /usr/local/include & /opt/local/include. 'openssl/ssl.h' is not present in either locations. But doing a brew install openssl says the following

Warning: openssl is a keg-only and another version is linked to opt.
Use `brew install --force` if you want to install this version

Doing a brew install openssl --force says

Warning: openssl-1.0.2k already installed, it's just not linked.

Doing brew link openssl --force also doesn't solve the issue.

Doig a which openssl returns the following:

/usr/local/bin/openssl

Please suggest

Linked or not, all installed Homebrew packages are reachable via link in /usr/local/opt. Additionally, when you install openssl via Homebrew, Homebrew tells you how to use the openssl libraries and headers.

~ nega@rust 15s
❯ brew install openssl
==> Downloading https://homebrew.bintray.com/bottles/openssl-1.0.2k.el_capitan.bottle.tar.gz
Already downloaded: /Users/nega/Library/Caches/Homebrew/openssl-1.0.2k.el_capitan.bottle.tar.gz
==> Pouring openssl-1.0.2k.el_capitan.bottle.tar.gz
==> Using the sandbox
==> Caveats
A CA file has been bootstrapped using certificates from the SystemRoots
keychain. To add additional certificates (e.g. the certificates added in
the System keychain), place .pem files in
  /usr/local/etc/openssl/certs

and run
  /usr/local/opt/openssl/bin/c_rehash

This formula is keg-only, which means it was not symlinked into /usr/local.

Apple has deprecated use of OpenSSL in favor of its own TLS and crypto libraries

If you need to have this software first in your PATH run:
  echo 'export PATH="/usr/local/opt/openssl/bin:$PATH"' >> ~/.zshrc

For compilers to find this software you may need to set:
    LDFLAGS:  -L/usr/local/opt/openssl/lib
    CPPFLAGS: -I/usr/local/opt/openssl/include
For pkg-config to find this software you may need to set:
    PKG_CONFIG_PATH: /usr/local/opt/openssl/lib/pkgconfig

==> Summary