CMake: restore PLAT so EXAMPLE/ and TESTING/ link the built library

[xiaoyeli]

Problem

The default CMake build fails (e.g. this CI run):

[  2%] Linking C executable pclinsol
/usr/bin/ld: cannot find -lsuperlu_mt: No such file or directory
make[2]: *** [EXAMPLE/CMakeFiles/pclinsol.dir/build.make:101: EXAMPLE/pclinsol] Error 1

Commit b121cf0 ("Add option THREAD_API …") replaced the PLAT cache variable with THREAD_API, and SRC/ now builds superlu_mt_PTHREAD and/or superlu_mt_OPENMP. But EXAMPLE/CMakeLists.txt and TESTING/MATGEN/CMakeLists.txt still do target_link_libraries(... superlu_mt${PLAT}). With PLAT unset this resolves to superlu_mt, which is not a target. CMake therefore passes a raw -lsuperlu_mt to the linker and adds no build dependency on the library, so linking fails.

Fix

The top-level CMakeLists.txt now sets PLAT from THREAD_API: _OPENMP for OPENMP, and _PTHREAD for PTHREAD or BOTH. The examples and tests then link the real library target again. No other files change.

Testing

Locally (GCC, reference BLAS), with the CI flags -Wall -Wextra -Wpedantic:

THREAD_API	build	ctest
PTHREAD (default, what CI runs)	OK	48/48 pass
BOTH	OK	48/48 pass
OPENMP	OK	48/48 pass on most runs; see note

Note: with THREAD_API=OPENMP, pztest_4_10_2_100000000_LA failed once in about 6 full runs (z_div: division by zero, info 10). It looks like a nondeterministic issue in the OpenMP code path itself, not in this build fix. Before this change that configuration did not link at all. CI does not exercise it.

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https://claude.ai/code/session_017GsYX7J15S6uvtUoRMWYwi

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