Always add third_party/libpng to include directories (thanks to Tobias Hansen).

Because private libpng headers are used by loadpng and
src/file/png_format.cpp.

CMakeLists.txt

@@ -90,6 +90,11 @@ endif()
 # Zlib generated zconf.h file
 include_directories(${CMAKE_BINARY_DIR}/third_party/zlib)
 
+# We need private libpng header files so it doesn't matter if we use
+# the shared version of libpng or not, we need to find header files in
+# "third_party/libpng"" directory.
+include_directories(${LIBPNG_DIR})
+
 # Search in the "cmake" directory for additional CMake modules.
 list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake)
 

src/CMakeLists.txt

@@ -49,7 +49,6 @@ if(USE_SHARED_LIBPNG)
   endif(PNG_FOUND)
 else()
   set(libs3rdparty ${libs3rdparty} libpng)
-  include_directories(${LIBPNG_DIR})
 endif()
 
 if(USE_SHARED_GIFLIB)

third_party/CMakeLists.txt

@@ -17,7 +17,6 @@ if(NOT USE_SHARED_ZLIB)
 endif()
 
 if(NOT USE_SHARED_LIBPNG)
-  include_directories(libpng)
   add_subdirectory(libpng)
 endif()