Externals: add hidapi

Current master (d17db57b9d4354752e0af42f5f33007a42ef2906) because
Android support was added after the current release (0.8.0-rc1).

Externals/hidapi/pc/hidapi-hidraw.pc.in

@@ -0,0 +1,10 @@
+prefix=@prefix@
+exec_prefix=@exec_prefix@
+libdir=@libdir@
+includedir=@includedir@
+
+Name: hidapi-hidraw
+Description: C Library for USB/Bluetooth HID device access from Linux, Mac OS X, FreeBSD, and Windows. This is the hidraw implementation.
+Version: @VERSION@
+Libs: -L${libdir} -lhidapi-hidraw
+Cflags: -I${includedir}/hidapi