CMake include_directories skips OS X frameworks - macos

Question

I'm using CMake 3.3.2 on OS X Yosemite. If I put a framework path into include_directories the generated Makefile doesn't include that directory. A short example:
cmake_minimum_required(VERSION 3.0)
project(testproj)
include_directories(/System/Library/Frameworks/OpenGL.framework)
add_library(testlib test.c)
The make output:
Scanning dependencies of target testlib
/Library/Developer/CommandLineTools/usr/bin/make -f CMakeFiles/testlib.dir/build.make CMakeFiles/testlib.dir/build
[ 50%] Building C object CMakeFiles/testlib.dir/test.c.o
/Library/Developer/CommandLineTools/usr/bin/cc -o CMakeFiles/testlib.dir/test.c.o -c /Users/wrar/test/test.c
I expected the include_directories command to have an effect on the compiler line, and as the official OPENGL_INCLUDE_DIR has the value I passed in the example, I expect it to be a correct value. What am I missing?

Answer 1

Turning my comments into an answer
Standard framework include paths are filtered by CMake
CMake has a lot of special handling for OS X Frameworks. One of it is to remove explicitly named framework include and library search paths (see this commit).
I agree that it's confusing that
find_package(OpenGL REQUIRED)
message(STATUS "OPENGL_INCLUDE_DIR: ${OPENGL_INCLUDE_DIR}")
include_directories(${OPENGL_INCLUDE_DIR})
add_library(...)
does return OPENGL_INCLUDE_DIR: /System/Library/Frameworks/OpenGL.framework but you don't find this path in the cc command line.
But this search path is implicit, meaning the compiler knows best where to find the framework's header files. So this filtering of the OS X framework include/library directories in CMake does reflect the special handling of the compiler/linker when working with frameworks.
How does the compiler find a framework's include file?
By the framework's name given as an include's directory name. See MAC Developer Library/Framework Programming Guide/Including Frameworks:
You include framework header files in your code using the #include directive. [...]
#include <Framework_name/Header_filename.h>
See cmake not finding gl.h on OS X for more details.
I've just tested your example and added e.g. #include <OpenGL/glext.h> to test.c and the header in the framework is found.
If you don't want the OpenGL standard framework to be found first (the default), see find_path documentation and look for CMAKE_FIND_FRAMEWORK.
Or if you have your own OpenGL distribution in /some/other/path - and you omit the use of find_package(OpenGL) - you can give include_directories(SYSTEM /some/other/path).