BUILD: fix FindMiniZip.cmake module to use pkg-config

On some distros the present logic will fail to detect
/usr/include/minizip/zip.h, use proper pkg-config call
to fix this. Regular cmake modules are broken and should
always be fixed to have some pkg-config logic, since it's the
only cross-distro standard to get correct library and include dirs.
BUILD: make MINIZIP_* vars overrideable
BUILD: add LIBRARY_DIRS to link directories

Although this is rarely necessary the libs could be in a
non-standard path and MINIZIP__LIBRARIES does not return
absolute paths.
Merge pull request #340 from hasufell/master

BUILD: fix FindMiniZip.cmake module to use pkg-config