diff --git a/Utilities/cmexpat/CMakeLists.txt b/Utilities/cmexpat/CMakeLists.txt
index 9a62b79af9..81dfee3124 100644
--- a/Utilities/cmexpat/CMakeLists.txt
+++ b/Utilities/cmexpat/CMakeLists.txt
@@ -6,6 +6,12 @@ ELSEIF(CMAKE_C_COMPILER_ID STREQUAL "PathScale")
   SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -woffall")
 ENDIF()
 
+# Activate POSIX APIs.
+if(CMAKE_SYSTEM_NAME MATCHES "^(Linux)$")
+  add_definitions(-D_DEFAULT_SOURCE -D_BSD_SOURCE)
+  string(APPEND CMAKE_REQUIRED_DEFINITIONS " -D_DEFAULT_SOURCE -D_BSD_SOURCE")
+endif()
+
 include(ConfigureChecks.cmake)
 configure_file(expat_config.h.cmake expat_config.h @ONLY)