Use system pnglite. Based on Gentoo Bugzilla

From: Azamat H. Hackimov <azamat.hackimov@gmail.com>

https://bugs.gentoo.org/show_bug.cgi?id=363395
---

 bam.lua                         |   33 +++++++++++++++++++++------------
 src/engine/client/graphics.cpp  |    2 +-
 src/tools/dilate.cpp            |    2 +-
 src/tools/tileset_borderfix.cpp |    2 +-
 4 files changed, 24 insertions(+), 15 deletions(-)


diff --git a/bam.lua b/bam.lua
index ce24cff..46b8e42 100644
--- a/bam.lua
+++ b/bam.lua
@@ -9,6 +9,7 @@ config = NewConfig()
 config:Add(OptCCompiler("compiler"))
 config:Add(OptTestCompileC("stackprotector", "int main(){return 0;}", "-fstack-protector -fstack-protector-all"))
 config:Add(OptLibrary("zlib", "zlib.h", false))
+config:Add(OptLibrary("pnglite", "pnglite.h", false))
 config:Add(OptLibrary("wavpack", "wavpack/wavpack.h", false))
 config:Add(SDL.OptFind("sdl", true))
 config:Add(FreeType.OptFind("freetype", true))
@@ -177,26 +178,34 @@ function build(settings)
 		settings.cc.includes:Add("src/engine/external/zlib")
 	end
 
+	-- build game components
+	engine_settings = settings:Copy()
+	server_settings = engine_settings:Copy()
+	client_settings = engine_settings:Copy()
+	launcher_settings = engine_settings:Copy()
+
+	if config.pnglite.value == true then
+		client_settings.link.libs:Add("pnglite")
+		if config.pnglite.include_path then
+			client_settings.cc.includes:Add(config.pnglite.include_path)
+		end
+		pnglite = {}
+	else
+		pnglite = Compile(settings, Collect("src/engine/external/pnglite/*.c"))
+		client_settings.cc.includes:Add("src/engine/external/pnglite")
+	end
+
 	if config.wavpack.value == true then
-		settings.link.libs:Add("wavpack")
+		client_settings.link.libs:Add("wavpack")
 		if config.wavpack.include_path then
-			settings.cc.includes:Add(config.wavpack.include_path)
+			client_settings.cc.includes:Add(config.wavpack.include_path)
 		end
 		wavpack = {}
 	else
 		wavpack = Compile(settings, Collect("src/engine/external/wavpack/*.c"))
-		settings.cc.includes:Add("src/engine/external/") --The header is wavpack/wavpack.h so include the external folder
+		client_settings.cc.includes:Add("src/engine/external/") --The header is wavpack/wavpack.h so include the external folder
 	end
 
-	-- build the small libraries
-	pnglite = Compile(settings, Collect("src/engine/external/pnglite/*.c"))
-
-	-- build game components
-	engine_settings = settings:Copy()
-	server_settings = engine_settings:Copy()
-	client_settings = engine_settings:Copy()
-	launcher_settings = engine_settings:Copy()
-
 	if family == "unix" then
 		if platform == "macosx" then
 			client_settings.link.frameworks:Add("OpenGL")
diff --git a/src/engine/client/graphics.cpp b/src/engine/client/graphics.cpp
index d1f0b8a..f654e87 100644
--- a/src/engine/client/graphics.cpp
+++ b/src/engine/client/graphics.cpp
@@ -20,7 +20,7 @@
 #endif
 
 #include <base/system.h>
-#include <engine/external/pnglite/pnglite.h>
+#include <pnglite.h>
 
 #include <engine/shared/config.h>
 #include <engine/graphics.h>
diff --git a/src/tools/dilate.cpp b/src/tools/dilate.cpp
index eb770a9..cbd7a3e 100644
--- a/src/tools/dilate.cpp
+++ b/src/tools/dilate.cpp
@@ -2,7 +2,7 @@
 /* If you are missing that file, acquire a complete release at teeworlds.com.                */
 #include <base/system.h>
 #include <base/math.h>
-#include <engine/external/pnglite/pnglite.h>
+#include <pnglite.h>
 
 typedef struct
 {
diff --git a/src/tools/tileset_borderfix.cpp b/src/tools/tileset_borderfix.cpp
index 0facb9a..ab36292 100644
--- a/src/tools/tileset_borderfix.cpp
+++ b/src/tools/tileset_borderfix.cpp
@@ -1,7 +1,7 @@
 /* (c) Magnus Auvinen. See licence.txt in the root of the distribution for more information. */
 /* If you are missing that file, acquire a complete release at teeworlds.com.                */
 #include <base/system.h>
-#include <engine/external/pnglite/pnglite.h>
+#include <pnglite.h>
 
 typedef struct
 {