[fluidsynth] Always install to "lib" rather than "lib64".

[BillyONeal]

Fixes this failure:

-- Performing post-build validation
There should be no pkgconfig directories outside of lib and debug/lib.
The following misplaced pkgconfig files were found:

    /mnt/vcpkg-ci/packages/fluidsynth_x64-linux/lib64/pkgconfig/fluidsynth.pc
    /mnt/vcpkg-ci/packages/fluidsynth_x64-linux/debug/lib64/pkgconfig/fluidsynth.pc

You can move the pkgconfig files with the following commands:

    file(INSTALL "${CURRENT_PACKAGES_DIR}/a/path/pkgconfig/name.pc" DESTINATION "${CURRENT_PACKAGES_DIR}/a/path/pkgconfig")
    file(REMOVE_RECURSE "${CURRENT_PACKAGES_DIR}/a/path/pkgconfig")

Found 1 error(s). Please correct the portfile:
    /agent/_work/2/s/ports/fluidsynth/portfile.cmake
-- Performing post-build validation done

/cc @autoantwort

[NancyLi1013]

Thanks for your fixes @BillyONeal.

There is another PR #20950 that tries to fix the same problem. But this PR also contains other changes.

The failures on arm64-windows:

FAILED: src/gentables-prefix/src/gentables-stamp/gentables-configure D:/buildtrees/fluidsynth/arm64-windows-dbg/src/gentables-prefix/src/gentables-stamp/gentables-configure 
cmd.exe /C "cd /D D:\buildtrees\fluidsynth\arm64-windows-dbg\src\gentables && D:\downloads\tools\cmake-3.21.1-windows\cmake-3.21.1-windows-i386\bin\cmake.exe D:/buildtrees/fluidsynth/src/d2fbc413bc-822d7b6699.clean/src/gentables -G Ninja -A Win32 -B D:/buildtrees/fluidsynth/arm64-windows-dbg/src/gentables && D:\downloads\tools\cmake-3.21.1-windows\cmake-3.21.1-windows-i386\bin\cmake.exe -E touch D:/buildtrees/fluidsynth/arm64-windows-dbg/src/gentables-prefix/src/gentables-stamp/gentables-configure"
CMake Error at CMakeLists.txt:13 (project):
  Generator

    Ninja

  does not support platform specification, but platform

    Win32

  was specified.


CMake Error: CMAKE_C_COMPILER not set, after EnableLanguage
-- Configuring incomplete, errors occurred!
See also "D:/buildtrees/fluidsynth/arm64-windows-dbg/src/gentables/CMakeFiles/CMakeOutput.log".
ninja: build stopped: subcommand failed.

I met the same problem when upgrading this port in PR #20978. I removed -G "${CMAKE_GENERATOR}" from the patch to fix this.