[gdk-pixbuf/build.fixes] meson.build: Don't use fallback dep for libpng unnecessarily

[Chun-wei Fan <fanchunwei src gnome org>]

commit 1fb8bd53dcb1bd40bc0c9c8537cd24fd9000eb4c
Author: Chun-wei Fan <fanchunwei src gnome org>
Date:   Thu Mar 7 18:05:13 2019 +0800

    meson.build: Don't use fallback dep for libpng unnecessarily
    
    We could have well found the headers and .lib's needed for libpng, so
    only use the fallback wrapper for libpng when we couldn't find the
    headers and .lib's.

 meson.build | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)
---
diff --git a/meson.build b/meson.build
index 6c5fa36cb..0e0799cb8 100644
--- a/meson.build
+++ b/meson.build
@@ -281,10 +281,12 @@ if get_option('png')
     # the --wrap-mode option. We don't directly call subproject() here because
     # that will bypass --wrap-mode and cause issues for distro packagers.
     # See: https://mesonbuild.com/Reference-manual.html#dependency
-    png_dep = dependency('', required: false, fallback: ['libpng', 'png_dep'])
-    if png_dep.found()
-      enabled_loaders += 'png'
-      loaders_deps += png_dep
+    if not png_dep.found()
+      png_dep = dependency('', required: false, fallback: ['libpng', 'png_dep'])
+      if png_dep.found()
+        enabled_loaders += 'png'
+        loaders_deps += png_dep
+      endif
     endif
   endif
 endif