Libpng config.tests: use pkg-config when available

Add pkg-config to the libpng usages as not all systems have the
symlink libpng.so -> libpng<version>.so (affected: NetBSD) that changes
with the version of the lib. If no-pkg-config is used, use -lpng as
before. Tested with FreeBSD 10.3, NetBSD 7.0.1 using png 1.6.21

Change-Id: I5c87f380c84da3d5c56c94da53adb900791c8caa
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

config.tests/unix/libpng/libpng.pro

@@ -1,3 +1,8 @@
 SOURCES = libpng.cpp
 CONFIG -= qt dylib
-LIBS += -lpng
+!contains(QT_CONFIG, no-pkg-config) {
+    CONFIG += link_pkgconfig
+    PKGCONFIG += libpng
+} else {
+    LIBS += -lpng
+}

src/3rdparty/png_dependency.pri

@@ -1,6 +1,14 @@
 contains(QT_CONFIG, system-png) {
-    unix|mingw: LIBS_PRIVATE += -lpng
+    unix|mingw {
-    else: LIBS += libpng.lib
+        !contains(QT_CONFIG, no-pkg-config) {
+            CONFIG += link_pkgconfig
+            PKGCONFIG_PRIVATE += libpng
+        } else {
+            LIBS_PRIVATE += -lpng
+        }
+    } else {
+        LIBS += libpng.lib
+    }
 } else: contains(QT_CONFIG, png) {
     include($$PWD/libpng.pri)
 }