Fix pkg-config {include,lib}dir paths.

[sebastic]

The includedir & libdir variables in the pkg-config file are incorrect when non-default paths are used using HDF4_INSTALL_INCLUDE_DIR & HDF4_INSTALL_LIB_DIR.

This is fixed by not hardcoding the subdirectories.

The resulting hdf.pc still has an issue, the private libs contain CMake targets:

Libs.private:  -lhdf -lmfhdf  -lm -l/usr/lib/x86_64-linux-gnu/libjpeg.so -lZLIB::ZLIB -llibaec::sz -llibaec::aec

I don't know how to fix this, so left this out of scope for this PR.
Hopefully someone more familiar with CMake knows how to resolve these targets.

[jhendersonHDF]

HDFGroup/hdf5#6220 may be of use if anyone wants to take this work on.