FIXED: crmath build failed on old compilers not knowing -march=x86-64-v3

Upstream core-math adds -march=x86-64-v3 unconditionally on x86_64.
This microarchitecture level is only known to gcc >= 11 and clang >= 12;
older compilers error out.  Drop the flag in our crmath.cmake layer when
check_c_compiler_flag() reports it is unsupported.  It is only a tuning
hint, so dropping it loses AVX2 tuning but not correctness.

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

Author: JanWielemaker

cmake/crmath.cmake

@@ -2,18 +2,30 @@
 # FetchContent_MakeAvailable() in Deps.cmake.  Here we adjust the
 # target to fit the libswipl build without modifying the sources.
 
+include(CheckCCompilerFlag)
+
 if(TARGET crmath)
   if(LIBSWIPL_TYPE STREQUAL "SHARED")
     set_property(TARGET crmath PROPERTY POSITION_INDEPENDENT_CODE 1)
   endif()
 
-  # Upstream hard-codes -O2, which would override our
-  # CMAKE_C_FLAGS_<CONFIG>.  Remove it so the project optimization
-  # flags apply, while keeping upstream's strict FP options
-  # (-fno-fast-math -ffp-contract=off).
+  # Adjust the compile options set by the upstream CMakeLists.txt:
+  #  - Remove -O2, which would override our CMAKE_C_FLAGS_<CONFIG>, so
+  #    the project optimization flags apply.  Upstream's strict FP
+  #    options (-fno-fast-math -ffp-contract=off) are kept.
+  #  - Remove -march=x86-64-v3 when the compiler does not understand
+  #    it.  This microarchitecture level is only known to gcc >= 11 and
+  #    clang >= 12; older compilers error out on it.  It is only a
+  #    performance hint, so dropping it merely loses AVX2 tuning.
   get_target_property(_crmath_options crmath COMPILE_OPTIONS)
   if(_crmath_options)
     list(REMOVE_ITEM _crmath_options -O2)
+    if("-march=x86-64-v3" IN_LIST _crmath_options)
+      check_c_compiler_flag(-march=x86-64-v3 HAVE_MARCH_X86_64_V3)
+      if(NOT HAVE_MARCH_X86_64_V3)
+	list(REMOVE_ITEM _crmath_options -march=x86-64-v3)
+      endif()
+    endif()
     set_target_properties(crmath PROPERTIES
 			  COMPILE_OPTIONS "${_crmath_options}")
   endif()