disable system issetugid on OS X since it is not fork-safe

Noticed while testing similar code for AIX.
ok beck@

configure.ac

@@ -226,6 +226,11 @@ AM_CONDITIONAL([HAVE_ARC4RANDOM_BUF],
 	   -a "x$HOST_OS" != xnetbsd \
 	   -a "x$ac_cv_func_arc4random_buf" = xyes])
 
+# overrides for issetugid implementations with known issues
+AM_CONDITIONAL([HAVE_ISSETUGID],
+       [test "x$HOST_OS" != xdarwin
+	  -a "x$ac_cv_func_issetugid" = xyes])
+
 AC_CACHE_CHECK([whether va_copy exists], ac_cv_have_va_copy, [
 	AC_LINK_IFELSE([AC_LANG_PROGRAM([[
 #include <stdarg.h>

crypto/Makefile.am

@@ -103,6 +103,9 @@ endif
 if HOST_HPUX
 libcompat_la_SOURCES += compat/issetugid_hpux.c
 endif
+if HOST_DARWIN
+libcompat_la_SOURCES += compat/issetugid_osx.c
+endif
 if HOST_WIN
 libcompat_la_SOURCES += compat/issetugid_win.c
 endif

crypto/compat/issetugid_osx.c

@@ -0,0 +1,16 @@
+/*
+ * issetugid implementation for OS X
+ * Public domain
+ */
+
+#include <unistd.h>
+
+/*
+ * OS X has issetugid, but it is not fork-safe as of version 10.10.
+ * See this Solaris report for test code that fails similarly:
+ * http://mcarpenter.org/blog/2013/01/15/solaris-issetugid%282%29-bug
+ */
+int issetugid(void)
+{
+	return 1;
+}