There's a merged pull request on the void-packages repository

wxPython4: fix cross build.
https://github.com/void-linux/void-packages/pull/28899

Description:
The PYTHON_CONFIG path was using /bin/python3-config, but the cross
toolchains currently lack usrmerge. Since the canonical location is
/usr/bin anyway, switching to /usr/bin is 100% correct.

Since we are here, also clean up how CFLAGS are being set.

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#### General
- [ ] This is a new package and it conforms to the [quality requirements](https://github.com/void-linux/void-packages/blob/master/Manual.md#quality-requirements)

#### Have the results of the proposed changes been tested?
- [ ] I use the packages affected by the proposed changes on a regular basis and confirm this PR works for me
- [ ] I generally don't use the affected packages but briefly tested this PR

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