Fix build with fluidsynth 1.1.9.

An issue exists in fluidsynth 1.1.9 where the pkgconfig file may contain wrong
paths.
A fix was submitted upstream but the minuet issue remains:
*never* rely only on pkgconfig files. This is the perfect example why the CMake
code should only use the paths obtained using pkgconfig as 'HINTS'.
Also rename the pkgconfig vars names to avoid another issue.