CMake CHECK_CXX_COMPILER_FLAG logic invalid

[matthewfeickert]

In PR #23 CMake logic checks were added to try to catch invalid CXXFLAGS on different platforms. I would argue these should be reverted as:

• The CMake code

qcdloop/CMakeLists.txt

Lines 34 to 39 in b778fa9

	foreach(fl ${CXX_FLAGS_TO_CHECK})
	CHECK_CXX_COMPILER_FLAG(${fl} COMPILER_SUPPORTS_${fl})
	if(COMPILER_SUPPORTS_${fl})
	set(CXX_FLAGS_PASSED "${CXX_FLAGS_PASSED} ${fl}" )
	endif()
	endforeach()

is wrong if there are any spaces in the flags in ${CXX_FLAGS_TO_CHECK}, which is valid and possible if you set CXXFLAGS and override these hardcoded options.

• The approach of removing invalid flags instead of failing is not desirable. A user should know when compilation options are invalid and should correct these from the environment or by passing explicit CMake options. (c.f. ENH: Add Linux ppc64le builds conda-forge/qcdloop-feedstock#1)

• It contains logic errors like

qcdloop/CMakeLists.txt

Line 26 in b778fa9

set(CXX_FLAGS_DEBUG_TO_CHECK "${CMAKE_CXX_FLAGS_DEBUG} -fsanitize=address ${CMAKE_CXX_FLAGS_TO_CHECK}")

(CMAKE_CXX_FLAGS_TO_CHECK hasn't been set yet, should be CXX_FLAGS_TO_CHECK).

Trying to guard users from incorrect compilation options ends up causing more harm than good, and it would be better to default to checking for the existence of environment variables like CXXFLAGS (as CMake already does) and in the absence of them set defaults based off of the compiler type.