Compile in C99 mode

* configure.ac: Require C99 flags. Remove -Wdeclaration-after-statement.

configure.ac

@@ -65,7 +65,7 @@ AC_CANONICAL_HOST
 AC_LIBTOOL_WIN32_DLL
 
 AC_PROG_INSTALL
-AC_PROG_CC
+AC_PROG_CC_C99
 gl_EARLY
 AC_PROG_CPP
 AC_PROG_SED
@@ -1524,8 +1524,7 @@ case "$GCC" in
     ## <http://lists.gnu.org/archive/html/guile-devel/2012-01/msg00487.html>.
 
     POTENTIAL_GCC_CFLAGS="-Wall -Wmissing-prototypes \
-      -Wdeclaration-after-statement -Wpointer-arith \
-      -Wswitch-enum -fno-strict-aliasing -fwrapv"
+      -Wpointer-arith -Wswitch-enum -fno-strict-aliasing -fwrapv"
     # Do this here so we don't screw up any of the tests above that might
     # not be "warning free"
     if test "${GUILE_ERROR_ON_WARNING}" = yes