Fix version detection in Findhidapi.cmake

[daschuer]

The first commit is the bug fix, taken from #4200, the second commit is for the improved messages.

[Be-ing]

This doesn't fix the incorrect #include path which was fixed already in #4200.

[Be-ing]

This has a misleading error message in case hidapi is too old:

-- Found hidapi: /usr/lib/x86_64-linux-gnu/libhidapi-libusb.so  
-- Found libhidapi version < 0.10.0
-- Linking internal libhidapi 0.10.1 statically

Found libhidapi version < 0.10.0 is misleading. While it is technically correct, we require 0.10.1 and this is what the user should be told. find_package(hidapi 0.10.1) as in #4200 does this.

[Be-ing]

This is error prone. I do not anticipate that anyone would remember to update this line when updating the vendored library.

[daschuer]

This doesn't fix the incorrect #include path which was fixed already in #4200.

For my understanding that was a regression by #4300 itself.

[Be-ing]

No, the Mixxx code has been wrong. #4200 fixes it.

[daschuer]

No, the Mixxx code has been wrong. #4200 fixes it.

Please explain how it has worked since than.

[Be-ing]

It worked because the build system fudged the include paths by treating /usr/include/hidapi as the include path. It does not do this for any other library; hidapi should not be an exception.

[daschuer]

This is the way it is used in the examples on https://github.com/libusb/hidapi and in many other Findhidapi.cmke implementations.
The PC file has the

Cflags: -I${includedir}/hidapi

We should use the library in the recommended way. This is by the way an extra topic not related to the bug fix in this PR.

[daschuer]

OK, all findings are resolved, merge?

[Be-ing]

I don't care which of this or #4200 gets merged. They both do the same thing. It was frustrating to have to review this.

[uklotzde]

I'm slightly in favor of merging #4200, because it is simpler and the error message is comprehensible for me. The special case handling is restricted to Findhidapi.cmake and does not creep into CMakeLists.txt.

[daschuer]

OK, here the full fix as discussed as second option.

Now a it utilizes PC_hidapi_VERSION and falls back reading the header if the PC file is not available.
The error message is generated using FindPackageHandleStandardArgs() correctly with the default error message.

In case of Ubuntu Bionic it prints now:

-- Could NOT find hidapi: Found unsuitable version "0.8.0-rc1", but required is at least "0.10.1" (found /usr/lib/x86_64-linux-gnu/libhidapi-libusb.so)
-- Linking internal libhidapi statically

[Be-ing]

I was not aware of the VERSION_VAR parameter of FindPackageHandleStandardArgs. That is the best solution.

[daschuer]

@Be-ing:
We have the policy to use merge commits!
It looks like you have changed you habit recently. #4172 #4121 #4163 #4058
Please use merge commits in future to not information like the relationship of commit messages to change in the code.