diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 98391741..8fe5cced 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -42,6 +42,10 @@ configure_file (
 
 add_definitions(-DVERSION_FILE_PRESENT -DFT2_BUILD_LIBRARY -DGPAC_DISABLE_VTT -DGPAC_DISABLE_OD_DUMP)
 
+if(CMAKE_SIZEOF_VOID_P EQUAL 8)
+  add_definitions(-DGPAC_64_BITS)
+endif()
+
 include_directories ("${PROJECT_SOURCE_DIR}")
 include_directories ("${PROJECT_SOURCE_DIR}/lib_ccx/")
 include_directories ("${PROJECT_SOURCE_DIR}/utf8proc/")