Bump up version number to 1.18.2
Update NEWS for 1.18.2 release
Workaround: Use PKG_CONFIG_PATH instead of PKG_CONFIG_LIBDIR for mingw32

Not sure why PKG_CONFIG_LIBDIR does not work with mingw32 cross
compiler. Android toolchain works with it. As a workaround, we use
PKG_CONFIG_PATH for mingw32.