CMake changes: Long double/double size detection, Android x86 and armv7 support, install pkg-config pc file

[ViliusSutkus89]

Hello,

This PR has the following changes:

• Generate pkg-config openlibm.pc file (issue #301)
• Support for Android armv7-a and Android x86
• Reworked long double size detection. Previously it was hardcoded, I've added a test which compares LDBL_MANT_DIG and DBL_MANT_DIG from float.h. Previously hardcoded cases did not cover Android x86 correctly. A simple compiler check should produce a reliable result.

Also, when we release, we need to update version number at the top of CMakeLists.txt file, otherwise the generated pkg-config openlibm.pc file will report the wrong version. I've set the version to 0.8.4 in advance. I don't really care about it, because I don't use pkg-config.pc file, I'm just saying what would be the consequences of not incrementing the version number. Where do we set the version number for pkg-config.pc file generated by autotools?

Once again, #299 will require a merge commit, because it's the same file we are editing, but we touched different lines, so we should be good.

Regards

[ViralBShah]

@ViliusSutkus89 Happy to give you commit access here. Please let me know if ok.

[ViliusSutkus89]

Sure. I can't be a full time maintainer, but I'll try to help whenever I can

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 39.02%. Comparing base (4b83beb) to head (c1e8462). Report is 7 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #304 +/- ## ========================================== + Coverage 38.24% 39.02% +0.78% ========================================== Files 233 233 Lines 6139 6144 +5 Branches 1607 1608 +1 ========================================== + Hits 2348 2398 +50 + Misses 3394 3393 -1 + Partials 397 353 -44 ```

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[ViliusSutkus89]

Updated PR with a patch to match version in CMake with the version in Autotools