Building libusb was also broken on GCC (and maybe Clang) on our
CMakeLists after upgrading to 1.0.24, but it was not being checked
because our 18.04 container had libusb installed on it.
This builds on the MinGW work from earlier and extends it to the rest of
the GNU toolchains. In addition we make use of pkg-config when present
to find libusb. pkg-config is preferrable because we can specify a
minimum required version.
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