Issue 49684 - AC_PROG_CC clobbers CFLAGS set by --enable-debug
    
    Bug description:
    Default CFLAGS and CXXFLAGS might be unset without --enable-debug.
    
    Fix description:
    * Provide default CFLAGS and CXXFLAGS that would be set by AC_PROG_CC
    otherwise.
    * Split compiler flags and preprocessor flags into separate variables so
    they are applied in a correct order.
    
    https://pagure.io/389-ds-base/issue/49684
    
    Reviewed by: mhonek (Thanks!)