Try to prevent direct use of exit(), malloc(), etc

Use `#pragma GCC poison` (supported since GCC 3, maybe earlier) when
compiling with GCC to help prevent direct uses being introduced for
functions which DOH provides a wrapper for.

Source/CParse/parser.y

@@ -25,6 +25,15 @@
 %{
 #define yylex yylex
 
+/* doh.h uses #pragma GCC posion with GCC to prevent direct calls to certain
+ * standard C library functions being introduced, but those cause errors due
+ * to checks like `#if defined YYMALLOC || defined malloc` in the bison
+ * template code.  We can't easily arrange to include headers after that
+ * template code, so instead we disable the problematic poisoning for this
+ * file.
+ */
+#define DOH_NO_POISON_MALLOC_FREE
+
 #include "swig.h"
 #include "cparse.h"
 #include "preprocessor.h"
@@ -34,6 +43,9 @@
 #undef alloca
 #define alloca Malloc
 
+#define YYMALLOC Malloc
+#define YYFREE Free
+
 /* -----------------------------------------------------------------------------
  *                               Externals
  * ----------------------------------------------------------------------------- */