libusb: Include PTHREAD_CFLAGS when linking libhidapi-libusb

m4/ax_pthread.m4 says that PTHREAD_CFLAGS needs to be provided to the
linker line as well as the compile line.  This enables static library
builds of hidapi.

Author: swt2c

libusb/Makefile.am

@@ -3,7 +3,7 @@ AM_CPPFLAGS = -I$(top_srcdir)/hidapi $(CFLAGS_LIBUSB)
 if OS_LINUX
 lib_LTLIBRARIES = libhidapi-libusb.la
 libhidapi_libusb_la_SOURCES = hid.c
-libhidapi_libusb_la_LDFLAGS = $(LTLDFLAGS)
+libhidapi_libusb_la_LDFLAGS = $(LTLDFLAGS) $(PTHREAD_CFLAGS)
 libhidapi_libusb_la_LIBADD = $(LIBS_LIBUSB)
 endif