Error out when mingw_startup() *and* NO_UNSETENV are active

The unsetenv code has no idea to update our environ_size, therefore
causing segmentation faults when environment variables are removed
without compat/mingw.c's knowing (MinGW's optimized lookup would try
to strcmp() against NULL in such a case).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.h

@@ -263,6 +263,10 @@ char *mingw_mktemp(char *template);
 char *mingw_getcwd(char *pointer, int len);
 #define getcwd mingw_getcwd
 
+#ifdef NO_UNSETENV
+#error "NO_UNSETENV is incompatible with the MinGW startup code!"
+#endif
+
 char *mingw_getenv(const char *name);
 #define getenv mingw_getenv
 int mingw_putenv(const char *namevalue);