* Add missing `add_subdirectory()` call for "cleanup" (#925)

Since `absl::Cleanup` is now public, it should also be included
in the `absl/CMakeLists.txt` file.

Signed-off-by: Christian Blichmann <cblichmann@google.com>

* Correctly install pkgconfig files under CMAKE_INSTALL_LIBDIR
Fixes #931

PiperOrigin-RevId: 366816645

* AbseilConfigureCopts.cmake: fix AppleClang detection

restore use of MATCHES in comparison with "Clang"; this was lost in:

commit 22771d471930ce88e1e75d0ca9dd8c65a7b0f895
  ...
  24e1f5f72756046f5265abf618e951c341f09b8d by Derek Mauro <dmauro@google.com>:

  Fixes failing CMake string comparisons
  https://cmake.org/cmake/help/latest/policy/CMP0054.html

fixes:
CMake Warning at absl/copts/AbseilConfigureCopts.cmake:61 (message):
  Unknown compiler:
  /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++.
  Building with no default flags

Fixes #930

PiperOrigin-RevId: 366879337

* Increment SOVERSION to 2103.0.1

Co-authored-by: Christian Blichmann <cblichmann@users.noreply.github.com>
Co-authored-by: Abseil Team <absl-team@google.com>