[CMake] include a file from the toolchain file

[Federico Kircheis]

As described on https://github.com/Kitware/CMake/ I'm writing here to 
check if the current behavior of cmake is really a bug.

I'm having following issues with toolchains files.

Here is my use case:
I have a `toolchain.gcc.cmake` and a `toolchain.clang.cmake` toolchain 
files.
Those files define variables for the gcc and clang compiler, and some 
settings (location of libraries, version numbers and so on) are common 
to both, and I would like to have a common `toolchain.cmake` toolchain 
file for that.

If I write (uncomment on of the includes)

-----
# toolchain.gcc.cmake

#include(common_settings.cmake)
#include(./common_settings.cmake)
#include(${CMAKE_CURRENT_SOURCE_DIR}/common_settings.cmake)
#include(${CMAKE_SOURCE_DIR}/common_settings.cmake)
#include(${CMAKE_HOME_DIRECTORY}/common_settings.cmake)

set(CMAKE_C_COMPILER gcc)
----

and execute (an empty CMakeLists.txt in the current directory is sufficient)

----
rm -rf build; mkdir build && (cd build && cmake 
-DCMAKE_TOOLCHAIN_FILE=../toolchain.gcc.cmake ..);
----

then cmake complains:
----
   include could not find load file:

 
/home/user/Workspace/toolchain/build/CMakeFiles/CMakeTmp/common_settings.cmake
Call Stack (most recent call first):
 
/home/user/Workspace/toolchain/build/CMakeFiles/3.12.3/CMakeSystem.cmake:6 
(include)
 
/home/user/Workspace/toolchain/build/CMakeFiles/CMakeTmp/CMakeLists.txt:2 
(project)


CMake Error at /usr/share/cmake-3.12/Modules/CMakeTestCCompiler.cmake:37 
(try_compile):
   Failed to configure test project build system.
Call Stack (most recent call first):
   CMakeLists.txt:3 (project)
----

As far as I've understood, cmake runs some internal test where, for 
example, `CMAKE_CURRENT_SOURCE_DIR` would not be the source directory of 
my code, and therefore 
`include(${CMAKE_CURRENT_SOURCE_DIR}/common_settings.cmake)` obviously 
fails.
Same holds for other variables.


The problem is that there is no way to set a path relative to the 
toolchain file, at least I was not able to find any.

The only possible fix is setting an absolute path, but this is not 
really a solution since it makes the toolchain not portable.

Is there anything else I could try?
Is there already an open issue? I was not able to find any on 
https://gitlab.kitware.com/cmake/cmake/issues/.

Thank you in advance

Federico