Log Message:
gcc49: don't use precompiled headers for libstdc++

this is a build time optimization that causes hangs on some machines
(netbsd-7/i386, but not netbsd-7/amd64). disable it globally instead
of guessing which are broken.

gcc{5,6} are already disabling it.

PR pkg/51352: Compilation error on lang/gcc49
PR pkg/50210: Compilation of gcc 4.9 and 5.2 hangs on NetBSD 6.1_STABLE