MinGW fixes

[FtZPetruska]

When working on updating the buildscript to work again on MinGW, I noticed the following issues:

• Despite passing the correct directory with TOOLCHAIN_DEPS_DIR, pkg-config is not able to find any .pc file.
• strndup is simply not available.

I addressed the dependency issues the following way:

• Make the minimum version 3.4, this is what is typically recommended as the bare minimum version to use. This also enables pkg_check_modules to search in prefixes specified in CMAKE_PREFIX_PATH.
• Make TOOLCHAIN_DEPS_DIR a cache entry. When it is passed through the command-line or via ExternalProject's CMAKE_ARGS, it is set as a cache entry already. Making it a cache entry when not provided avoids weird CMake shadowing issues.
• Set CMAKE_PREFIX_PATH instead of PKG_CONFIG_PATH. This has the advantage of letting CMake use the correct lib paths for the current system (e.g. when a system uses lib64 instead of lib).

For strndup, I added a wrapper function dupstring that either forwards the call strndup, or use a fallback implementation. The check for whether strndup is available or not is made in CMake, and passed to the wrapper source file only.

[FtZPetruska]

Another Windows-related issue (which applies to Msys2): vita-headers fails build due to the Makefile generated by vita-libs-gen. The issue comes from the command $(AR) cru $@ $? which fails with the error arm-vita-eabi-ar: Argument list too long when too many object files are used. This is especially an issue with SceLibc as the object file list hits 71620 characters while the limit is around 26k characters.

The workaround for this is to use a response file which contains all the object filenames, and pass it to ar using @file. Since Makefile generated by vita-libs-gen are only meant to be called with arm-vita-eabi-ar, we are guaranteed to have support for this feature.

Fixes #127

[d3m3vilurr]

CI issue could be checked by @Princess-of-Sleeping
I'll share to her.

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@FtZPetruska hm... changed cmake version may make a trouble. can you cross check it?

[FtZPetruska]

CI issue seems to come from:

- uses: actions/checkout@v3

Since psprela is a git submodule, CI should checkout the repo using:

- uses: actions/checkout@v3
  with:
    submodules: true

Checking previous CI runs, it seems that it also was an issue on older CMake versions, but not treated as an error for compatibility:

(run #179)
 CMake Warning (dev) at CMakeLists.txt:11 (add_subdirectory):
  The source directory

    /home/runner/work/vita-toolchain/vita-toolchain/psp2rela

  does not contain a CMakeLists.txt file.

  CMake does not support this case but it used to work accidentally and is
  being allowed for compatibility.

[Princess-of-Sleeping]

Can you also update workflow with this PR?

[FtZPetruska]

Can you also update workflow with this PR?

Made the change in 51a2142

[Princess-of-Sleeping]

@d3m3vilurr I think good for now.