ext/gd/config.m4: don't forget GDLIB_CFLAGS in feature tests

In commit 85e5635, a feature test for the various libgd image formats
was added. That test however erroneously omits the GDLIB_CFLAGS (from
pkg-config) during compilation. This can lead to build failures and
therefore false negatives from the test.

Here, we add $GDLIB_CFLAGS to $CFLAGS for the duration of the test.

Moreover, we replace $GD_SHARED_LIBADD with $GDLIB_LIBS in the same
test. The variable $GD_SHARED_LIBADD is actually empty when we try to
use it. Fortunately(?), the library flags are appended elsewhere, so
this was not causing a problem. But of course it's better to
explicitly do the right thing.

Closes phpGH-12019

ext/gd/config.m4

@@ -149,7 +149,9 @@ dnl display a warning but eventually return normally, making a simple link
 dnl or run test insufficient.
 AC_DEFUN([PHP_GD_CHECK_FORMAT],[
   old_LIBS="${LIBS}"
-  LIBS="${LIBS} ${GD_SHARED_LIBADD}"
+  LIBS="${LIBS} ${GDLIB_LIBS}"
+  old_CFLAGS="${CFLAGS}"
+  CFLAGS="${CFLAGS} ${GDLIB_CFLAGS}"
   AC_MSG_CHECKING([for working gdImageCreateFrom$1 in libgd])
   AC_LANG_PUSH([C])
   AC_RUN_IFELSE([AC_LANG_SOURCE([
@@ -180,6 +182,7 @@ int main(int argc, char** argv) {
     AC_MSG_RESULT([no])
   ])
   AC_LANG_POP([C])
+  CFLAGS="${old_CFLAGS}"
   LIBS="${old_LIBS}"
 ])