cmake: add -Wall to CFLAGS

[npajkovsky]

No description provided.

[Sashan]

makes sense. thanks.

[esyr]

I would prefer to see changes in the source files as a separate preparatory commit, but I guess it's fine here. Otherwise, the only meaningful change needed is the directive ordering, everything else is optional.

[esyr]

Please move it after cmake_minimum_required and project directives and add CACHE and FORCE parameters (so it can be overridden from the command line).

[esyr]

Also, I'm not sure that cl supports -Wall, it should be /Wall or something like that, rather, so, probably, it would better be something like that:

if(WIN32)
    set(CFLAGS_WARNINGS "/Wall" CACHE STRING "Compiler options for warnings")
else()
    set(CFLAGS_WARNINGS "-Wall" CACHE STRING "Compiler options for warnings")
endif()

set(set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${CFLAGS_WARNINGS}"
    CACHE STRING "C compiler options" FORCE)