mingw fixes:

1) mingw needs an #include to have access to mkdir.

2) It needs to always #include port.h (this is an identical
bit of code, in configure.ac, that I have in other opensource
projects for mingw support.)

3) I moved some code from port.cc to port.h, so I didn't have
to add logic to link in port.cc for mingw.

Last change before new release!  (*knock on wood*)  Submitting
TBR so I can get the release out today.  This isn't exactly a
trivial change, so I'm chary to submit TBR, but it's pretty
isolated to windows and mingw, and I've tested on those
platforms to make sure they compile and all tests pass.

DELTA=70  (37 added, 30 deleted, 3 changed)


Revision created by MOE tool push_codebase.
MOE_MIGRATION=2823


git-svn-id: https://gflags.googlecode.com/svn/trunk@54 6586e3c6-dcc4-952a-343f-ff74eb82781d