Change CMake library name back to portaudio from PortAudio

[donarturo11]

Fixes #939 .

EDIT (added by RossBencina):

The purpose of this patch is to revert back to the CMake library name portaudio (all lowercase) that is used in our current V19.7 release. This avoids breaking things for all users of V19.7 CMake support. Note that this is not a git revert but rather a manual renaming.

[philburk]

I looked at the PR and it seems limited to changing the case of "PortAudio" to "portaudio".
That is consistent with the request by Ross and @donarturo11 agreed to in #939.

But the CMake build for Ubuntu and MacOS is failing with:

/home/runner/work/portaudio/portaudio/qa/loopback/src/paqa.c:45:10: fatal error: portaudio.h: No such file or directory

For MInGW:

Cannot specify link libraries for target "PortAudio" which is not built by

[RossBencina]

Thanks for the PR. It's failing CI, I'm not sure why, could you take a look please? It seems like there are still some active occurrences of PortAudio in the file, maybe that's it? Please do a case-sensitive search for PortAudio and review all of the results.

[donarturo11]

@RossBencina @philburk Could You retry, please? I merged yet another commits. I fixed cases in tests, examples and CMakeLists.txt. On my CI is now running successful.

[RossBencina]

@dechamps @dmitrykos other CMake users: Please could you weigh in on this. Right now I see no reason to break V19.7 users by renaming the library but maybe I'm missing something.

[philburk]

I think this change can be considered as a bug fix for an earlier breaking change.
Lacking any objections from CMake experts, I think we should do this.

[RossBencina]

I would have liked to hear some feedback from CMake users, but there has been none for over a month.

Without further information this PR effectively fixes a regression relative to V19.7. If there was a valid reason to rename the library to camel case I don't what it is, and without that information I have no way to evaluate the benefit of not merging this PR.

So we're going to go ahead and merge it.