Fix CMake install dir defaults on macOS/Windows when CMAKE_INSTALL_LIBDIR is specified

[justsmth]

Description of changes:

On macOS and Windows, the fallback defaults for CMAKE_INSTALL_INCLUDEDIR and CMAKE_INSTALL_BINDIR were gated behind a single elseif(NOT DEFINED CMAKE_INSTALL_LIBDIR) check. If CMAKE_INSTALL_LIBDIR was passed on the command line (e.g. -DCMAKE_INSTALL_LIBDIR=lib), the condition was false and the other two variables never got set — causing install(DIRECTORY ... DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}) to fail with "install DIRECTORY given no DESTINATION!".

This changes the elseif to an else block that sets each variable independently when not already defined.

Call-outs:

Only affects the macOS/Windows code path (the NOT WIN32 AND NOT APPLE branch already uses GNUInstallDirs which handles everything). The gRPC integration script (run_grpc_integration.sh) hits this because aws_lc_build always passes -DCMAKE_INSTALL_LIBDIR=lib.

Testing:

Verified the CMake configure step succeeds on macOS with -DCMAKE_INSTALL_LIBDIR=lib. The gRPC integration build (run_grpc_integration.sh) no longer errors during the AWS-LC configure phase.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.12%. Comparing base (a75e930) to head (2ddce7d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3069      +/-   ##
==========================================
+ Coverage   77.95%   78.12%   +0.16%     
==========================================
  Files         689      689              
  Lines      122506   122506              
  Branches    17095    17100       +5     
==========================================
+ Hits        95505    95706     +201     
+ Misses      26102    25901     -201     
  Partials      899      899              

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