Appease clang -Wgnu-statement-expression

We use GNU statement expressions in crypto/md32_common.h, surrounded
by checks that GNU C is indeed used to compile.  It seems that clang,
at least on Linux, pretends to be GNU C, therefore finds the statement
expressions and then warns about them.

The solution is to have clang be quiet about it.

Reviewed-by: Rich Salz <rsalz@openssl.org>