Skip executable malloc() test on PowerPC OpenBSD

[Josh Elsasser]

On OpenBSD, memory returned by malloc() is never marked executable.
However it seems that on OpenBSD/macppc with some CPUs, this
restriction isn't enforced. Unfortunately, this doesn't mean that it
will work with perfect reliability. On my mac mini g4, the configure
test would only fail about half of the time.

Forcing the test to fail allows a working method to be reliably
chosen.

diff --git m4/codeexec.m4 m4/codeexec.m4
index ff51972..144a6da 100644
--- m4/codeexec.m4
+++ m4/codeexec.m4
@@ -39,6 +39,11 @@ AC_DEFUN([FFCALL_CODEEXEC],
          dnl test program loops endlessly.
          ffcall_cv_codeexec=no
          ;;
+       powerpc--openbsd*)
+         dnl On this platform, malloc()ed memory is not marked as executable,
+         dnl but the test program may not reliably detect this.
+         ffcall_cv_codeexec=no
+         ;;
        *)
          AC_TRY_RUN(GL_NOCRASH
            [#include <sys/types.h>