Fix macOS OpenMP duplicate-runtime abort in homebrew build

esys libs were compiled with llvm clang's -fopenmp (linking
.../opt/llvm/lib/libomp.dylib) while their openblas dependency links the
standalone 'libomp' formula (.../opt/libomp/lib/libomp.dylib). Loading two
different libomp.dylib files in one process triggers "OMP: Error #15 ...
already initialized" and an Abort trap: 6 at the first parallel region, so
examples crash under plain python3 (run-escript only papered over it with
KMP_DUPLICATE_LIB_OK=TRUE).

Put the libomp-formula lib dir first in ld_extra so the -lomp added by
-fopenmp resolves to the same physical runtime openblas uses, leaving one
OpenMP runtime in the process. Paths use HOMEBREW_PREFIX so the fix works on
both Apple Silicon (/opt/homebrew) and Intel (/usr/local).

Also drop the redundant -lz from ld_extra (zlib is already linked via
zlib_libs=['z']) to silence "ld: warning: ignoring duplicate libraries: '-lz'".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: LutzGross

scons/templates/homebrew_options.py

@@ -170,7 +170,22 @@
 scotch_libs = ['ptscotch', 'ptscotcherr', 'scotch', 'scotcherr']
 
 # Linker flags - include LLVM libc++ for macOS arm64
-ld_extra = ["-L/opt/homebrew/opt/llvm/lib", "-L/opt/homebrew/opt/llvm/lib/c++", "-lz", "-Wl,-rpath,/opt/homebrew/opt/llvm/lib/c++"]
+#
+# IMPORTANT (macOS OpenMP duplicate-runtime fix):
+# Homebrew's openblas links the standalone 'libomp' formula
+# (/opt/homebrew/opt/libomp/lib/libomp.dylib), while clang's -fopenmp would
+# otherwise pull in LLVM's own copy (/opt/homebrew/opt/llvm/lib/libomp.dylib).
+# Loading two different libomp.dylib files in one process triggers
+# "OMP: Error #15 ... already initialized" and an Abort trap: 6 at the first
+# parallel region. We put the libomp-formula lib dir *first* so that the
+# -lomp added by -fopenmp resolves to the same physical libomp that openblas
+# uses, leaving exactly one OpenMP runtime in the process.
+# Note: zlib is linked via zlib_libs=['z'] above; do not repeat -lz here or the
+# linker warns "ignoring duplicate libraries: '-lz'".
+ld_extra = ["-L" + HOMEBREW_PREFIX + "/opt/libomp/lib",
+            "-L" + HOMEBREW_PREFIX + "/opt/llvm/lib", "-L" + HOMEBREW_PREFIX + "/opt/llvm/lib/c++",
+            "-Wl,-rpath," + HOMEBREW_PREFIX + "/opt/libomp/lib",
+            "-Wl,-rpath," + HOMEBREW_PREFIX + "/opt/llvm/lib/c++"]
 
 # Optional features
 # sympy not available in Homebrew - requires virtual environment installation