[gdk-pixbuf/build.fixes: 2/2] meson.build: Don't use fallback dep for libpng unnecessarily

[Emmanuele Bassi <ebassi src gnome org>]

commit e2a6f6401b11f08c31662c17be26d340effe89b5
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