libfolly.pc: Fix for cross compilation (#1226)

Summary:
Changes libfolly.pc to a more standard format.

Allows overriding of the variables by pkgconfig.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

I'm not sure of anything that uses pkgconfig to find libfolly but might as well fix it.
Pull Request resolved: https://github.com/facebook/folly/pull/1226

Reviewed By: simpkins

Differential Revision: D17415620

Pulled By: yfeldblum

fbshipit-source-id: d015c3e83783b1c5e9ac48a64ad35c0c07120dbd

CMake/libfolly.pc.in

 prefix=@CMAKE_INSTALL_PREFIX@
-libdir=@CMAKE_INSTALL_FULL_LIBDIR@
-includedir=@CMAKE_INSTALL_FULL_INCLUDEDIR@
+exec_prefix=@CMAKE_INSTALL_PREFIX@
+libdir=${exec_prefix}/@CMAKE_INSTALL_LIBDIR@
+includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@
 
 Name: libfolly
 Description: Facebook (Folly) C++ library