Make build glue accept libpng 1.6.x and don't hardcode suffix

--- CMakeLists.txt.orig	2017-05-27 09:58:05 UTC
+++ CMakeLists.txt
@@ -582,9 +582,10 @@ else()
 	set(LIBZIP_LIBRARY libzip)
 endif()
 
-# FindPNG does a few things we don't want. So do it ourselves. Fixed to libpng17
-find_path(PNG_PNG_INCLUDE_DIR NAMES "libpng17/png.h")
-find_library(PNG_LIBRARY NAMES png17 libpng17)
+find_package(PkgConfig)
+pkg_check_modules(PNG libpng>=1.6)
+set(PNG_LIBRARY ${PNG_LIBRARIES})
+set(PNG_PNG_INCLUDE_DIR ${PNG_INCLUDE_DIRS})
 find_package(PackageHandleStandardArgs)
 find_package_handle_standard_args(PNG REQUIRED_VARS PNG_LIBRARY PNG_PNG_INCLUDE_DIR)
 if (PNG_FOUND)
--- Core/Screenshot.cpp.orig	2017-05-27 09:58:05 UTC
+++ Core/Screenshot.cpp
@@ -21,7 +21,7 @@
 #ifdef USING_QT_UI
 #include <QtGui/QImage>
 #else
-#include <libpng17/png.h>
+#include "png.h"
 #include "ext/jpge/jpge.h"
 #endif
 
--- ext/native/image/png_load.cpp.orig	2017-05-27 09:58:05 UTC
+++ ext/native/image/png_load.cpp
@@ -5,7 +5,7 @@
 #ifdef USING_QT_UI
 #include <QtGui/QImage>
 #else
-#include "libpng17/png.h"
+#include "png.h"
 #endif
 
 #include "png_load.h"
--- Core/TextureReplacer.cpp.orig	2017-05-27 09:58:05 UTC
+++ Core/TextureReplacer.cpp
@@ -16,7 +16,7 @@
 // https://github.com/hrydgard/ppsspp and http://www.ppsspp.org/.
 
 #ifndef USING_QT_UI
-#include <libpng17/png.h>
+#include "png.h"
 #endif
 
 #include <algorithm>