Fix scope of PM_ACTUAL_LIB_NAME

[rrrapha]

Setting an alternative name for the static library via PM_STATIC_LIB_NAME has been broken in commit e878378.

This change fixes it by setting PM_ACTUAL_LIB_NAME in the current scope AND the parent scope.

It can be tested like this:

cmake -DBUILD_SHARED_LIBS=OFF -DPM_STATIC_LIB_NAME=test .
make

This should print PortMidi Library name: test and produce libtest.a.

[rbdannenberg]

Thanks! I had to get Claude to explain CMake scope rules to understand this. That seems like a real design flaw in CMake. Rather than pulling these changes, I added comments and fixes locally because without comments or a deeper understanding of CMake, the logic seems really strange. Maybe that's why it got broken to begin with.