Support old compilers

Some old compilers do not use C99 as their default C language variant,
for example gcc 4.3.x, which is used on some old machines. When such a
compiler is used, by default, ULLONG_MAX is not visible. Adding
-std=gnu99 is needed to make this definition visible.

In autoconf speak, this translates into the need of using
AC_PROC_CC_STDC instead of AC_PROG_CC. See
http://www.gnu.org/software/autoconf/manual/autoconf-2.65/html_node/C-Compiler.html
for more details.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/configure.ac
===================================================================
--- a/configure.ac
+++ b/configure.ac
@@ -17,7 +17,7 @@
 AC_SYS_LARGEFILE
 
 # Checks for programs.
-AC_PROG_CC
+AC_PROG_CC_STDC
 AC_PROG_MAKE_SET
 LT_INIT
 AC_PROG_YACC