Pass compiler flags directly to scons as build arguments

The error persists: "Checking for C++ header file cmath... no"

Root cause: Setting ENV variables wasn't sufficient because scons has
its own configuration system that doesn't automatically pick up
environment variables properly.

Solution: Pass compiler flags directly to scons as command-line arguments
using its cc_flags and cxx_flags parameters:

1. cc_flags/cxx_flags accept compiler flag strings
2. Include -isysroot to point to SDK
3. Include -stdlib=libc++ for standard library
4. Explicitly add C++ header path: -I<SDK>/usr/include/c++/v1

This directly tells Cantera's scons build system where to find standard
C++ headers like <cmath>, bypassing any environment variable issues.

This is the approach used by other Homebrew formulas that build with
scons on macOS.

Author: sbryngelson

packaging/homebrew/mfc.rb

@@ -38,20 +38,20 @@ def install
       # Install Cantera build dependencies (including scons)
       system venv/"bin/pip", "install", "cython", "numpy", "ruamel.yaml", "packaging", "scons"
 
-      # Configure Cantera build
-      # Set compiler environment variables for scons
-      ENV["CC"] = ENV.cc
-      ENV["CXX"] = ENV.cxx
-      ENV["CFLAGS"] = "-isysroot#{MacOS.sdk_path}"
-      ENV["CXXFLAGS"] = "-isysroot#{MacOS.sdk_path}"
-
+      # Configure Cantera build - pass compiler and SDK paths to scons
+      # Build flags that include SDK path and standard C++ include directories
+      sdk_path = MacOS.sdk_path
+      cxx_flags = "-isysroot#{sdk_path} -stdlib=libc++ -I#{sdk_path}/usr/include/c++/v1"
+      
       # Run scons with the venv's Python so it can find installed packages
       system venv/"bin/python", "-m", "SCons", "build",
              "python_package=y",
              "f90_interface=n",
              "system_sundials=y",
              "system_yamlcpp=y",
              "system_fmt=n",
+             "cc_flags=#{cxx_flags}",
+             "cxx_flags=#{cxx_flags}",
              "extra_inc_dirs=#{Formula["sundials"].opt_include}:#{Formula["yaml-cpp"].opt_include}",
              "extra_lib_dirs=#{Formula["sundials"].opt_lib}:#{Formula["yaml-cpp"].opt_lib}",
              "prefix=#{libexec}/cantera",