Renamed __EMX__ (internal define of the gcc port under OS/2) to OS2

following the same idea as "MSVC vs WIN32". Additionally the src/os/emx/
directory was renamed to src/os/os2/ for consistency.

Background information:
1. The src/os/os2/ stuff was created by a repository copy
   so we're both keep the history and sill be able
   to checkout older Apache releases.
2. The __EMX__ check in ap_config.h was replaced by
   a OS2 check while src/Configure now sets CFLAGS
   to "-DOS ..". Same is done for the Unix platforms,
   so make no difference here just for OS2.
3. The src/os/os2 is now also references from within
   src/Configure to reflect the change.

Submitted by: Brian Havard
Reviewed by: Martin Kraemer, Ralf S. Engelschall
Finalized, cleaned up and applied by: Ralf S. Engelschall


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@81848 13f79535-47bb-0310-9956-ffa450edef68