[gdk-pixbuf/build.fixes: 14/14] meson.build: Don't use fallback dep for libpng unnecessarily

[Emmanuele Bassi <ebassi src gnome org>]

commit 4fee71121e24e06bd9ab41c11ea64cde5fed02b4
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 0e17cdbcf..45d878c07 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