Need an autoconf macro that detects if -m64 is a valid compiler option

Question

I have code that I want to compile on all unix systems, but if -m64 i available and it works, I want the configure script to use it. How do I get autoconf to check to see if -m64 works and, if so, use it?

Answer 1

my_save_cflags="$CFLAGS"
CFLAGS=-m64
AC_MSG_CHECKING([whether CC supports -m64])
AC_COMPILE_IFELSE([AC_LANG_PROGRAM([])],
    [AC_MSG_RESULT([yes])]
    [AM_CFLAGS=-m64],
    [AC_MSG_RESULT([no])]
)
CFLAGS="$my_save_cflags"
AC_SUBST([AM_CFLAGS])

Using AM_CFLAGS to add -m64 to the build assumes automake (or the use of AM_CFLAGS in your own non-automade makefiles.)