Remove dead or useless header checks from cmake and autoconf

On Windows, this improves clean cmake configuration time on my
workstation from 1m58s to 1m32s, which is pretty significant.  There's
probably more that can be done here, but this is the low hanging fruit.

Eric volunteered to regenerate ./configure for me.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187209 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/Errno.cpp

@@ -14,8 +14,6 @@
 #include "llvm/Support/Errno.h"
 #include "llvm/Config/config.h"     // Get autoconf configuration settings
 #include "llvm/Support/raw_ostream.h"
-
-#if HAVE_STRING_H
 #include <string.h>
 
 #if HAVE_ERRNO_H
@@ -75,5 +73,3 @@ std::string StrError(int errnum) {
 
 }  // namespace sys
 }  // namespace llvm
-
-#endif  // HAVE_STRING_H