diff --git a/CMakeLists.txt b/CMakeLists.txt
index c8a2525b..e7b398b4 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -121,13 +121,16 @@ CHECK_TYPE_SIZE("long" SIZEOF_LONG)
 CHECK_TYPE_SIZE("long long" SIZEOF_LONG_LONG)
 
 
-CHECK_C_COMPILER_FLAG(-std=c11 HAVE_STD_C11)
+# GNU dialects, not the strict ISO ones: -std=c11 defines __STRICT_ANSI__, which
+# makes newlib hide the POSIX functions used all over the code (strdup,
+# strcasecmp, strncasecmp, strtok_r, nanosleep).
+CHECK_C_COMPILER_FLAG(-std=gnu11 HAVE_STD_C11)
 IF(HAVE_STD_C11)
-    SET(CMAKE_C_FLAGS "-std=c11 ${CMAKE_C_FLAGS}")
+    SET(CMAKE_C_FLAGS "-std=gnu11 ${CMAKE_C_FLAGS}")
 ELSE()
-    CHECK_C_COMPILER_FLAG(-std=c99 HAVE_STD_C99)
+    CHECK_C_COMPILER_FLAG(-std=gnu99 HAVE_STD_C99)
     IF(HAVE_STD_C99)
-        SET(CMAKE_C_FLAGS "-std=c99 ${CMAKE_C_FLAGS}")
+        SET(CMAKE_C_FLAGS "-std=gnu99 ${CMAKE_C_FLAGS}")
     ENDIF()
 ENDIF()