Check if the compiler supports c99

[Carlos Eduardo Rodrigues Diógenes]

Hi,

I'm not a experiencied user/programmer in the unix world, but I start to
maintaim the gnome-mag code sometime ago, so I'm still have some
difficulties in understand some concepts of autoconf and how to search
for information, so I think that ask it here is not so bad ;-).

I need to use round () in a part of the code and I saw that in GCC I
must pass the -std=c99 option to the compiler and the -lm option to the
linker.

I yet put it in my configure.in:

        AC_CHECK_LIB(m, round, [M_LIBS="-lm"], AC_MSG_ERROR([libm is
        required to build gnome-mag. This lib comes with GNU C libraries
        (glibc) and you can find the lastest version from
        http://www.gnu.org/software/libc/]))
        AC_SUBST(M_LIBS)
        
This work for me check if the function round is present in libm, but if
I call this function in my code without the option -std=c99 to the
compiler, the code does not work properly. I found a middle solution
that is declare round () as an extern function.

But, for me this is not the best solution. If I could check if the
compiler supports the -std=c99 option I can use it to compile the code
and only declare round () as extern if the -std=c99 is not present but
the round function is in the libm.

I thinked in use AC_COMPILE_IFELSE, but don't realize how this can be
used to achieve this goal.

Any help will be very appreciated.

Thanks,
-- 
Carlos Eduardo Rodrigues Diógenes