I have posted a web page that describes (in very great detail) how to compile Gimp 2.9, using MinGW on 32-bit Windows. I noticed Partha's site some time ago, and decided that I would provide more detail, even for those who never heard of MinGW or compiling. This is in the hope that some newbies might want to try the program, which might help with bug reports.
The page has nothing about 64-bit. Can't help you there. My approach uses files from "Gimp Portable" latest version (now 2.8.2), most (not all) of which are over-written during compilation of 2.9. In particular, since Gimp Portable now includes Python, preconfigured to work with Gimp, it is possible to compile 2.9 with Python disabled. Then, at runtime, 2.9 finds the Python from Portable 2.8 and uses it. Also, I did not find it necessary to use external Perl.
The compiled 2.9 appears to be portable, probably because it is self-contained. I haven't looked at this in great detail, but if compiled using my method and file structure, I can copy out the necessary folder, and launch it via Windows command (rather than MinGW shell). My page describes this.
Note: Awhile back, there was a color management problem in 2.9, apparently due to a Gegl bug. That bug has been fixed. Currently, there is color management (needs testing, of course). Also, the program is not as slooow as it used to be. Needs improvement, but is better.http://robertallgeyer.com/content/speci ... p-2.9.html