Autoconf protype breaks GCC builtin inlining

[Ferenc Wágner]

Hi,

Since ceil() can be inlined when optimization is enabled, I use

AC_SEARCH_LIBS([ceil], [m], , [AC_MSG_ERROR([ceil not found])])
AC_SUBST([m_LIBS], [$LIBS])

to link against libm only when necessary (that is, when ceil is not
inlined).  However, it does not work because GCC says in config.log:

conftest.c:76:6: warning: conflicting types for built-in function 'ceil'
 char ceil ();
      ~~~~

and inlining is disabled, thus ceil becomes an undefined reference
without -lm.  However, in the actual code where math.h is #included, the
above warning does not happen, ceil is inlined and I unnecessarily link
with -lm.  How could I overcome this?

Please Cc me, I'm not subscribed.
-- 
Thanks,
Feri