cmake: Allow overriding flags in compiler/linker flag checks

[hebasto]

The APPEND_{CPP,C,CXX,LD}FLAGS flags are now considered when checking compiler/linker flags.

Fixes #279.

[fanquake]

Reading https://cmake.org/cmake/help/latest/variable/CMAKE_TRY_COMPILE_PLATFORM_VARIABLES.html, one of the first things it says is:

This variable should not be set by project code. It is meant to be set by CMake's platform information modules for the current toolchain, or by a toolchain file when used with CMAKE_TOOLCHAIN_FILE.

So I think this needs some documentation, or an explanation of what it's doing.

[hebasto]

Reading https://cmake.org/cmake/help/latest/variable/CMAKE_TRY_COMPILE_PLATFORM_VARIABLES.html, one of the first things it says is:

This variable should not be set by project code. It is meant to be set by CMake's platform information modules for the current toolchain, or by a toolchain file when used with CMAKE_TOOLCHAIN_FILE.

So I think this needs some documentation, or an explanation of what it's doing.

All CMake checks are implemented using the try_compile() command, which internally creates a tiny temporary test project. This PR applies to that test project the same approach as has been used since #184.

Setting the non-cache CMAKE_TRY_COMPILE_PLATFORM_VARIABLES variable before the project() command ensures that it will be available in all contexts mentioned in https://cmake.org/cmake/help/latest/variable/CMAKE_TRY_COMPILE_PLATFORM_VARIABLES.html. In that case, I can't see any drawbacks to not following "This variable should not be set by project code." statement.

[fanquake]

The APPEND_{CPP,C,CXX,LD}FLAGS flags are now considered

Note that this also isn't just append flags. DCMAKE_CXX_FLAGS_RELEASE weren't being included either.

[fanquake]

Note that this also isn't just append flags. DCMAKE_CXX_FLAGS_RELEASE weren't being included either.

Testing this branch it seems like they are also still being ignored? The test case from #279 is still broken.

[hebasto]

Note that this also isn't just append flags. DCMAKE_CXX_FLAGS_RELEASE weren't being included either.

Testing this branch it seems like they are also still being ignored? The test case from #279 is still broken.

It is possible to override only using APPEND_* flags.

[hebasto]

An explaining comment has been added per @fanquake's offline request.

[theuni]

I've been playing with this all day and it's.. a mess.

From my tests I don't see a better way to get the flags appended, so utACK that I guess :(

For @fanquake's complaint, it seems this is our problem: https://gitlab.kitware.com/cmake/cmake/-/issues/19512

I'm able to get build_type flags into the try_compiles by moving the default build type selection up and setting CMAKE_TRY_COMPILE_CONFIGURATION:

diff --git a/CMakeLists.txt b/CMakeLists.txt
index fe6eab74f01..27e96e5b278 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -73,6 +73,10 @@ set(CMAKE_CXX_EXTENSIONS OFF)

 list(APPEND CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake/module)

+include(ProcessConfigurations)
+set_default_config(RelWithDebInfo)
+set (CMAKE_TRY_COMPILE_CONFIGURATION ${CMAKE_BUILD_TYPE})
+
 #=============================
 # Configurable options
 #=============================
@@ -457,9 +461,6 @@ else()
   )
 endif()

-include(ProcessConfigurations)
-set_default_config(RelWithDebInfo)
-
 # Redefine/adjust per-configuration flags.
 target_compile_definitions(core_interface_debug INTERFACE
   DEBUG

@hebasto Mind having a look at that? Maybe it'd make sense to set that flag as part of set_default_config instead? Either way, it seems it's necessary to move the default selection up towards the beginning of the file.