Use check_symbol_exists() instead of check_function_exists().

This is mostly a revert of 477031c03b.
Now that we are not setting `CMAKE_C_FLAGS`, the check can work
correctly and it helps `pcc` (portable c compiler) make it further
along--though it still doesn't produce usable results (see #427 for the
details).

src/os/env.c

@@ -5,7 +5,7 @@
 #include "os/os.h"
 #include "misc2.h"
 
-#ifdef HAVE_CRT_EXTERNS_H
+#ifdef HAVE__NSGETENVIRON
 #include <crt_externs.h>
 #endif