libusb: require pkg-config support

An alternative approach to show how much cleaner the pure pkg-config
way is.

This changes the discovery procedures for libusb-1.0 and libusb-0.1,
making them depend on pkg-config being properly installed and
configured, including the necessary build host configuration for the
cross-builds (see
http://www.flameeyes.eu/autotools-mythbuster/pkgconfig/cross-compiling.html)

It should make it possible to compile OpenOCD without changes and
extra effort on GNU/Linux, FreeBSD users would need to supply a .pc
file for their libusb implementation or add LIBUSB1_LIBS and
LIBUSB1_CFLAGS to the configure environment.

Change-Id: I826e378dd1e0d101a549a573b2c63212a7e00b64
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1467
Reviewed-by: Xiaofan <xiaofanc@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Tested-by: Spencer Oliver <spen@spen-soft.co.uk>

README

@@ -210,6 +210,7 @@ You'll also need:
 
 - make
 - libtool
+- pkg-config >= 0.23 (or compatible)
 
 Additionally, for building from git:
 
@@ -218,7 +219,8 @@ Additionally, for building from git:
 - texinfo
 
 USB-based adapters depend on libusb-1.0 and some older drivers require
-libusb-0.1 or libusb-compat-0.1.
+libusb-0.1 or libusb-compat-0.1. A compatible implementation, such as
+FreeBSD's, additionally needs the corresponding .pc files.
 
 USB-Blaster, ASIX Presto, OpenJTAG and ft2232 interface adapter
 drivers need either one of:
@@ -272,6 +274,16 @@ e.g. for cross-building for Windows 32-bit with MinGW on Debian:
 
   ./configure --host=i686-w64-mingw32 [options]
 
+To make pkg-config work nicely for cross-compiling, you might need an
+additional wrapper script as described at
+
+  http://www.flameeyes.eu/autotools-mythbuster/pkgconfig/cross-compiling.html
+
+This is needed to tell pkg-config where to look for the target
+libraries that OpenOCD depends on. Alternatively, you can specify
+*_CFLAGS and *_LIBS environment variables directly, see "./configure
+--help" for the details.
+
 Parallel Port Dongles
 ---------------------