Add C++ include path directly to cxx_flags for scons checks

Issue: scons' configuration checks still couldn't find <cmath> even though
we were passing extra_inc_dirs with the C++ header path.

Root cause: Cantera's scons configuration checks don't use extra_inc_dirs
during the check phase - that parameter is only used during the actual
build. The checks use the cc_flags and cxx_flags directly.

Solution: Add -I<SDK>/usr/include/c++/v1 directly to the cxx_flags
parameter so it's included in scons' test compiles:

  cxx_flags="-isysroot<SDK> -stdlib=libc++ -I<SDK>/usr/include/c++/v1"

This ensures the C++ header path is available during scons' "Checking for
C++ header file cmath..." test.

Author: sbryngelson

packaging/homebrew/mfc.rb

@@ -55,12 +55,13 @@ def install
 
       # Run scons, explicitly passing compiler flags as Cantera scons variables
       # Cantera uses cc_flags/cxx_flags, not CCFLAGS/CXXFLAGS
-      # This ensures they apply to scons' configuration test compiles
+      # Include C++ header path directly in cxx_flags for scons' configuration checks
+      cxx_flags_with_includes = "#{ENV.fetch("CXXFLAGS", nil)} -I#{cxx_inc_path}"
       system venv/"bin/python", "-m", "SCons", "build",
              "CC=#{ENV.cc}",
              "CXX=#{ENV.cxx}",
              "cc_flags=#{ENV.fetch("CFLAGS", nil)}",
-             "cxx_flags=#{ENV.fetch("CXXFLAGS", nil)}",
+             "cxx_flags=#{cxx_flags_with_includes}",
              "python_package=y",
              "f90_interface=n",
              "system_sundials=y",