Externals: add hidapi

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

Externals/hidapi/hidtest/Makefile.am

@@ -0,0 +1,20 @@
+AM_CPPFLAGS = -I$(top_srcdir)/hidapi/
+
+## Linux
+if OS_LINUX
+noinst_PROGRAMS = hidtest-libusb hidtest-hidraw
+
+hidtest_hidraw_SOURCES = hidtest.cpp
+hidtest_hidraw_LDADD = $(top_builddir)/linux/libhidapi-hidraw.la
+
+hidtest_libusb_SOURCES = hidtest.cpp
+hidtest_libusb_LDADD = $(top_builddir)/libusb/libhidapi-libusb.la
+else
+
+# Other OS's
+noinst_PROGRAMS = hidtest
+
+hidtest_SOURCES = hidtest.cpp
+hidtest_LDADD = $(top_builddir)/$(backend)/libhidapi.la
+
+endif