libpng: disable building of executables

They are not packaged here. Upstream backport.

Speeds up compilation and avoids a zlib hack.

Refreshed patches.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

libs/libpng/patches/010-png-exec.patch

@@ -0,0 +1,38 @@
+From 28c0f8895e4ac270b56b5c7e8089dd2417bc4e3c Mon Sep 17 00:00:00 2001
+From: Alex Gaynor <alex.gaynor@gmail.com>
+Date: Sun, 20 May 2018 18:46:32 -0400
+Subject: [PATCH] cmake: Add an option to enable/disable building of
+ executables
+
+Add the CMake option PNG_EXECUTABLES (on by default) in order to
+allow or disallow the building of non-essential executable programs
+associated with libpng.
+
+Contributed-by: Alex Gaynor <alex.gaynor@gmail.com>
+Contributed-by: Cosmin Truta <ctruta@gmail.com>
+Signed-off-by: Cosmin Truta <ctruta@gmail.com>
+---
+ CMakeLists.txt | 6 ++++--
+ 1 file changed, 4 insertions(+), 2 deletions(-)
+
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -53,7 +53,8 @@ endif()
+ # COMMAND LINE OPTIONS
+ option(PNG_SHARED "Build shared lib" ON)
+ option(PNG_STATIC "Build static lib" ON)
+-option(PNG_TESTS  "Build libpng tests" ON)
++option(PNG_EXECUTABLES "Build libpng executables" ON)
++option(PNG_TESTS "Build libpng tests" ON)
+ 
+ # Many more configuration options could be added here
+ option(PNG_FRAMEWORK "Build OS X framework" OFF)
+@@ -747,7 +748,7 @@ if(PNG_TESTS AND PNG_SHARED)
+   png_add_test(NAME pngimage-full COMMAND pngimage OPTIONS --exhaustive --list-combos --log FILES ${PNGSUITE_PNGS})
+ endif()
+ 
+-if(PNG_SHARED)
++if(PNG_SHARED AND PNG_EXECUTABLES)
+   add_executable(pngfix ${pngfix_sources})
+   target_link_libraries(pngfix png)
+   set(PNG_BIN_TARGETS pngfix)