Fix Fypp chemistry boolean interpretation

The issue was that CMake was passing -D chemistry=False (string) to Fypp,
but Fypp interprets the string "False" as truthy (non-empty string).
This caused Fypp to generate code that tried to use m_thermochem even when
MFC_CHEMISTRY=OFF, leading to compilation errors.

Solution: Pass 0/1 instead of False/True to Fypp, which correctly interprets
as boolean false/true.

Also explicitly import join_path in package.py for clarity.

Author: sbryngelson

CMakeLists.txt

@@ -45,9 +45,10 @@ else()
 endif()
 
 # Chemistry configuration
-set(_chem_str "False")
+# Pass 0/1 to Fypp so it interprets correctly as boolean (not string "False"/"True")
+set(_chem_str "0")
 if (MFC_CHEMISTRY)
-    set(_chem_str "True")
+    set(_chem_str "1")
 endif()
 
 # Optional: mechanism file name/path for Cantera

packaging/spack/package.py

@@ -6,6 +6,7 @@
 from spack_repo.builtin.build_systems.cmake import CMakePackage
 
 from spack.package import *
+from spack.util.path import join_path
 
 
 class Mfc(CMakePackage):