Fixes issue #1879.

Directives were not directly expanded, but since their
content ended up in makefiles, further expansion would
take place there.  This prevents such artifacts by
restricting the set of characters that may be used in
a directive value.

To build the list of safe characters I went through the
contents of /usr/lib/pkgconfig and extracted LDFLAGS
and CFLAGS information, so hopefully this is a
reasonable default to get started.

R=rsc
CC=golang-dev
https://golang.org/cl/4532092