Issue 49684 - AC_PROG_CC clobbers CFLAGS set by --enable-debug
    
    Bug Description:
    AC_PROG_CC macro sets CFLAGS/CXXFLAGS environment variable to ` -g -O2`
    for GCC if CFLAGS/CXXFLAGS are not defined.
    
    Fix Description:
    Use an empty default for CFLAGS and CXXFLAGS instead.
    
    https://pagure.io/389-ds-base/issue/49684
    
    Reviewed by: mreynolds (Thanks!)