@nikic $GD_JPEG is always empty here, so JPEG support would never be available (same for WebP and XPM). A possible solution would be:

 ext/gd/config.m4 | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/ext/gd/config.m4 b/ext/gd/config.m4
index bb55c5fd94..9eb593f3b1 100644
--- a/ext/gd/config.m4
+++ b/ext/gd/config.m4
@@ -54,7 +54,7 @@ AC_DEFUN([PHP_GD_WEBP],[
 
 AC_DEFUN([PHP_GD_JPEG],[
   if test "$PHP_JPEG" != "no"; then
-    PKG_CHECK_MODULES([JPEG], [libjpeg])
+    PKG_CHECK_MODULES([JPEG], [libjpeg], [JPEG_FOUND=true])
     PHP_EVAL_LIBLINE($JPEG_LIBS, GD_SHARED_LIBADD)
     PHP_EVAL_INCLINE($JPEG_CFLAGS)
   fi
@@ -146,7 +146,7 @@ dnl enable the support in bundled GD library
     GDLIB_CFLAGS="$GDLIB_CFLAGS -DHAVE_LIBWEBP"
   fi
 
-  if test -n "$GD_JPEG"; then
+  if test -n "$JPEG_FOUND"; then
     AC_DEFINE(HAVE_GD_JPG, 1, [ ])
     GDLIB_CFLAGS="$GDLIB_CFLAGS -DHAVE_LIBJPEG"
   fi

I wonder, though, why we have an extra check here. It seems to me that we could define HAVE_GD_JPG and set GDLIB_CFLAGS right away after PKG_CHECK_MODULES([JPEG], [libjpeg]).

Thanks for pointing this out. I've changed this to set the defines directly in the checking functions. I've also replaced the GDLIB_CFLAGS with AC_DEFINEs. While these are only strictly necessary for the libgd build, I think that handling them via config.h is easier.

I've also dropped the feature checks in case an external libgd is used. In that case features are detected in a different way, by looking at symbols in the library. Whether libjpeg etc are present when PHP is compiled should not be relevant in that case.

While these are only strictly necessary for the libgd build, I think that handling them via config.h is easier.

ACK. However, ext/gd/libgd/gd_webp.c is not ready for that, since it includes gd.h (which includes config.h) only if HAVE_LIBWEBP is defined. So we'd also need something like:

 ext/gd/libgd/gd_webp.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/ext/gd/libgd/gd_webp.c b/ext/gd/libgd/gd_webp.c
index 51295797f5..bcd8008eab 100644
--- a/ext/gd/libgd/gd_webp.c
+++ b/ext/gd/libgd/gd_webp.c
@@ -1,10 +1,11 @@
-#ifdef HAVE_LIBWEBP
 #include <stdio.h>
 #include <math.h>
 #include <string.h>
 #include <stdlib.h>
 #include "gd.h"
 #include "gdhelpers.h"
+
+#ifdef HAVE_LIBWEBP
 #include "webp/decode.h"
 #include "webp/encode.h"

BTW: is there a particular reason why our Travis builds don't use --with-webp?

I've added these changes and also added --with-webp on Travis. Let's see if it runs into any issues.

I dropped this, because it does not appear to be in use.