plan9port: modify the builder to use the INSTALL script.

plan9port ships with an INSTALL script. This commit modifies the
builder to use the script instead of a custom build script. The
commit also adds a patch to build fontsrv, which is otherwise
omitted from the build.
plan9port: Do not fetch X11 build dependencies if building on darwin.
Merge pull request #2266 from vu3rdd/master

plan9port: modify the builder to use the INSTALL script.