drop standalone OpenMP dep; let qblas_dep propagate it

quaddtype's own sources don't use OpenMP (no #pragma omp or omp_*
calls). Both quaddtype's meson.build and qblas's meson.build called
dependency('openmp'), and qblas_dep also propagated the OpenMP dep
through its 'dependencies:' list. The same OpenMP instance therefore
appeared twice in quaddtype's compile-args closure.

On Apple's clang++ that double inclusion left an orphan -Xpreprocessor
in the args stream. Meson's ninja rule appends dependency-generation
flags ('-MD -MQ -MF ...') after $ARGS, so the orphan -Xpreprocessor
paired with -MD on the next line. clang++ then passed -MD to the
preprocessor verbatim, the preprocessor rejected it as unknown, and
every C++ compile failed - sinking the macos-15 wheel build.

qblas_dep already brings OpenMP transitively when QBLAS is enabled.
When QBLAS is disabled (Windows / -Ddisable_quadblas=true) we don't
need OpenMP at all because none of quaddtype's own sources use it.
Removing the second dependency('openmp') makes the closure single-
sourced and eliminates the duplicate compile args.

Author: SwayamInSync

meson.build

@@ -119,11 +119,14 @@ npymath_lib = c.find_library('npymath', dirs: npymath_path)
 
 dependencies = [py_dep, qblas_dep, sleef_dep, sleefquad_dep, npymath_lib]
 
-# Add OpenMP dependency (optional, for threading)
-openmp_dep = dependency('openmp', required: false, static: false)
-if openmp_dep.found()
-    dependencies += openmp_dep
-endif
+# OpenMP is not used directly by any source in this package (#pragma omp
+# is absent); it only matters because qblas's static lib has OpenMP
+# objects baked in. qblas_dep already propagates the OpenMP requirement
+# transitively, so adding a second dependency('openmp') here would put
+# OpenMP into the compile-args closure twice. On Apple's clang++ that
+# duplication leaves an orphan '-Xpreprocessor' in $ARGS which then
+# pairs with '-MD' from the dep-gen flags ('-Xpreprocessor -MD'), and
+# the preprocessor rejects -MD as unknown - failing all C++ compiles.
 
 # compiler flags for QBLAS compatibility
 if not is_windows