Make implicit function definition (in julia_init.c) an error (#936)

This is an easy thing to get wrong. Someone more familiar with Julia than
with C is not likely to recognize the significance of this error.

I've also almost never seen this intentionally ignored in the wild, so I
don't expect many false positives.

Author: topolarity

src/juliaconfig.jl

@@ -54,7 +54,8 @@ end
 
 function cflags()
     flags = IOBuffer()
-    print(flags, "-O2 -std=gnu99")
+    print(flags, " -Werror-implicit-function-declaration")
+    print(flags, " -O2 -std=gnu99")
     include = shell_escape(julia_includedir())
     print(flags, " -I", include)
     if Sys.isunix()