Add the libusb dependency to the pkg-config file.

When linking dynamically, the shared library contains a reference to all
external dependencies, and the linker can easily resolve them. However
when linking statically, all external dependencies have to be specified
explicitly. This rule also applies to dependencies that are not exposed
through the public api.

The pkg-config Requires.private field is used to support both static and
dynamic linking correctly.

configure.ac

@@ -47,6 +47,7 @@ AM_CONDITIONAL([OS_WIN32], [test "$os_win32" = "yes"])
 PKG_CHECK_MODULES([LIBUSB], [libusb-1.0], [have_libusb=yes], [have_libusb=no])
 if test "$have_libusb" = "yes"; then
 	AC_DEFINE([HAVE_LIBUSB], [1], [libusb support])
+	AC_SUBST([DEPENDENCIES], [libusb-1.0])
 fi
 
 # Checks for IrDA support.

libdivecomputer.pc.in

@@ -6,5 +6,6 @@ includedir=@includedir@
 Name: libdivecomputer
 Description: A library for communication with various dive computers.
 Version: @VERSION@
+Requires.private: @DEPENDENCIES@
 Libs: -L${libdir} -ldivecomputer
 Cflags: -I${includedir}/libdivecomputer