cmake: allow to use pkg-config for windows builds as well

cmake: allow to use pkg-config for windows builds as well

If the user has an appropriate pkg-config wrapper,
e.g. i686-w64-mingw32-pkg-config (or building natively on mingw and
has pkg-config or pkg-config-lite installed), it can be used to locate
libusb-1.0 properly. For cross-compiling
-DPKG_CONFIG_EXECUTABLE=`which i686-w64-mingw32-pkg-config`
needs to be explicitly set.

It still doesn't prevent pkg-config-less operation, i.e. the user can
specify something like
-DLIBUSB_INCLUDE_DIR=~/i686-w64-mingw32-root/usr/include/libusb-1.0
and the build will succeed even when no pkg-config executable is
present.

Signed-off-by: Paul Fertser <fercerpav@gmail.com>