cmake: disable some gnu extensions
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 98881b5..e1754e5 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -1,5 +1,6 @@
add_library(git2internal OBJECT)
set_target_properties(git2internal PROPERTIES C_STANDARD 90)
+set_target_properties(git2internal PROPERTIES C_EXTENSIONS OFF)
if(DEPRECATE_HARD)