Fix pyjnius linking with Qt bootstrap libmain

[lazycat11]

When using the Qt bootstrap, ndk-build generates the main library as libmain_.so, for example libmain_arm64-v8a.so. pyjnius links with -lmain, so the linker cannot find the ABI-suffixed library.

This change creates a libmain.so alias before building pyjnius and adds the bootstrap libs/ directory to LDFLAGS.

Tested with Buildozer / python-for-android on arm64-v8a.

[T-Dynamos]

Wouldn't this increase app size? I think it would be better to patch pyjnius to use -lmain_ instead.

[lazycat11]

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