[gegl] configure: make JPEG and PNG a strict dependency

[Øyvind Kolås <ok src gnome org>]

commit 0736fed752f5ce98077a95a5f24db56f3de2e123
Author: Sven Neumann <sven gimp org>
Date:   Sat Nov 21 23:13:07 2015 +0100

    configure: make JPEG and PNG a strict dependency
    
    The build will fail in the generation of the example images
    if JPEG support is missing. Refuse to build without usable
    JPEG and PNG libraries.

 configure.ac |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index a9c9397..8456bdd 100644
--- a/configure.ac
+++ b/configure.ac
@@ -809,6 +809,10 @@ AM_CONDITIONAL(HAVE_JPEG, test "$jpeg_ok" = "yes")
 
 AC_SUBST(LIBJPEG)
 
+if test "$jpeg_ok" != "yes"; then
+   AC_MSG_ERROR([Could not find a usable JPEG library with header files])
+fi
+
 
 ##################
 # Check for libpng
@@ -828,6 +832,9 @@ AM_CONDITIONAL(HAVE_PNG, test "$have_libpng" = "yes")
 AC_SUBST(PNG_CFLAGS) 
 AC_SUBST(PNG_LIBS) 
 
+if test "$have_libpng" != "yes"; then
+   AC_MSG_ERROR([Could not find a usable PNG library with header files])
+fi
 
 ###################
 # Check for librsvg