--- a/tools/src/CMakeLists.txt
+++ b/tools/src/CMakeLists.txt
@@ -23,6 +23,8 @@
 # Based heavily upon the libftdi cmake setup.

 set(INSTALL_DEFAULT_BINDIR "bin" CACHE STRING "Appended to CMAKE_INSTALL_PREFIX")
+
+set(CMAKE_C_STANDARD 99)

 if(MSVC)
 add_library(libgetopt_static STATIC