diff --git a/src/lib_ccx/CMakeLists.txt b/src/lib_ccx/CMakeLists.txt
index 3a36f53f..998cca96 100644
--- a/src/lib_ccx/CMakeLists.txt
+++ b/src/lib_ccx/CMakeLists.txt
@@ -11,6 +11,20 @@ AUX_SOURCE_DIRECTORY("${PROJECT_SOURCE_DIR}/gpacmp4/" SOURCEFILE)
 #AUX_SOURCE_DIRECTORY("${PROJECT_SOURCE_DIR}/libpng/" SOURCEFILE)
 add_library(ccx ${SOURCEFILE})
 
+
+if (MINGW OR CYGWIN)
+    set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -DGPAC_CONFIG_WIN32")
+endif (MINGW OR CYGWIN)
+
+if (${CMAKE_SYSTEM_NAME} MATCHES "Linux")
+    set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -DGPAC_CONFIG_LINUX")
+endif (${CMAKE_SYSTEM_NAME} MATCHES "Linux")
+
+if (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
+    set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -DGPAC_CONFIG_DARWIN")
+endif (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
+
+
 FILE(GLOB HeaderFiles *.h)
 file(WRITE ccx.pc "prefix=${CMAKE_INSTALL_PREFIX}\n"
                   "includedir=\${prefix}/include\n"