doc: exclude gegl from documentation conditionally

Doxyfile is now created by configure. The gegl directory is now only
included in the doxygen input when configured with `--enable-gegl`.

configure.ac

@@ -281,6 +281,7 @@ AC_ARG_ENABLE(gegl,
     [enable GEGL based code in build (yes|no, default=no)])
 )
 
+DOXYGEN_EXCLUDED=
 if eval "test x$enable_gegl = xyes"; then
   AC_MSG_NOTICE([Checking whether gegl-0.4 or gegl-0.3 is available])
   # Prioritize gegl-0.4
@@ -289,13 +290,17 @@ if eval "test x$enable_gegl = xyes"; then
     PKG_CHECK_MODULES(GEGL, gegl-0.3, [GEGL_VERSION=0.3])
 ])
 AC_SUBST(GEGL_VERSION)
+else
+  DOXYGEN_EXCLUDED="../../gegl/"
 fi
 AM_CONDITIONAL(ENABLE_GEGL, test "x$enable_gegl" = "xyes")
+AC_SUBST(DOXYGEN_EXCLUDED)
 
 # Set pkg-config variables before generation.
 AC_SUBST(PKG_CONFIG_REQUIRES)
 
 AC_CONFIG_FILES([
+  doc/Doxyfile
   doc/Makefile
   gegl/libmypaint-gegl-]libmypaint_api_platform_version()[.pc:gegl/libmypaint-gegl.pc.in
   gegl/Makefile

doc/.gitignore

@@ -1,2 +1,3 @@
+Doxyfile
 doxygen/
 build/

doc/Doxyfile → doc/Doxyfile.in

@@ -859,7 +859,7 @@ RECURSIVE              = YES
 # Note that relative paths are relative to the directory from which doxygen is
 # run.
 
-EXCLUDE                = ../../tests ../../glib/
+EXCLUDE                = ../../tests ../../glib/ @DOXYGEN_EXCLUDED@
 
 # The EXCLUDE_SYMLINKS tag can be used to select whether or not files or
 # directories that are symbolic links (a Unix file system feature) are excluded