Fix linux CI, use C99 mode (#12)

src/milagro_internals.nim

@@ -1,3 +1,9 @@
+# Milagro should be compiled for C99.
+# This statement checks to see if we're using a backend other then C, and if not, passes C99.
+# We would just check for C except Nim only defines the other backends.
+when not defined(cpp) or defined(objc) or defined(js):
+  {.passC: "-std=c99".}
+
 import strutils
 from os import DirSep