For FreeBSD, don't define _M in nasty_macros.hpp

Because FreeBSD uses _M in its <sys/types.h>, and it is hard to avoid
including that header, only define _M to NASTY_MACRO for other operating
systems.  This fixes almost 2000 unexpected test failures.

Discussed with Eric Fiselier.

llvm-svn: 339794
diff --git a/libcxx/test/support/nasty_macros.hpp b/libcxx/test/support/nasty_macros.hpp
index 76d8ab0..97a17bb 100644
--- a/libcxx/test/support/nasty_macros.hpp
+++ b/libcxx/test/support/nasty_macros.hpp
@@ -22,7 +22,11 @@
 #define _J NASTY_MACRO
 #define _K NASTY_MACRO
 #define _L NASTY_MACRO
+// Because FreeBSD uses _M in its <sys/types.h>, and it is hard to avoid
+// including that header, only define _M for other operating systems.
+#ifndef __FreeBSD__
 #define _M NASTY_MACRO
+#endif
 #define _N NASTY_MACRO
 #define _O NASTY_MACRO
 #define _P NASTY_MACRO