Fix MinGW builds of Premake

[starkos]

What does this PR do?

Gets Premake's bootstrap makefile to work properly with MinGW (64-bit only). This is an attempt to resurrect PR #1111.

Closes #559.

# To build with bootstrap script
set CC=gcc
mingw32-make -f Bootstrap.mak mingw PLATFORM=x64

# To build with Premake
premake5 --cc=mingw gmake2
mingw32-make config=debug_x64

How does this PR change Premake's behavior?

A few additions were made to Premake to improve support for MinGW specifically. Those were added by @tdesveauxPKFX in #1111, I've maintained them as is.

Anything else we should know?

I've successfully built Premake using the 64-bit version of MinGW using these changes. I was not able to build with the 32-bit version, though that may have been a local setup issue; I didn't pursue it. Since, best I could tell, we aren't currently able to build with MinGW at all this seems like an incremental win.

I left @tdesveauxPKFX's commits intact so he would get some credit for the fix, since he did the bulk of the work on this one.

Two unit tests fail with the MinGW build:

1. base_os.remove_ReturnsTrue_OnFile gets an "Access denied" error when it tries to delete a temporary file
2. premake_binmodules.testExample fails with "module 'example' not found"

Did you check all the boxes?

• Focus on a single fix or feature; remove any unrelated formatting or code changes
• Add unit tests showing fix or feature works; all tests pass (minimal code changes)
• Mention any related issues
• Follow our coding conventions
• Minimize the number of commits

You can now support Premake on our OpenCollective. Your contributions help us spend more time responding to requests like these!

[samsinsane]

Seems good, for what it's worth, it seems like AppVeyor has MinGW support, might be possible to have CI jobs for MinGW.

[starkos]

Rebased to master, waiting for checks to complete.

[starkos]

HTTP tests are failing with "Bad Gateway", but it varies which platform fails each time I run it. If I can't get a clean pass I may just merge.