Add configure flag `with-chemistry`

configure.ac

@@ -932,6 +932,26 @@ case "$with_cooling" in
    ;;
 esac
 
+#  chemistry function
+AC_ARG_WITH([chemistry],
+   [AS_HELP_STRING([--with-chemistry=<function>],
+      [chemistry function @<:@none, grackle default: none@:>@]
+   )],
+   [with_chemistry="$withval"],
+   [with_chemistry="none"]
+)
+case "$with_chemistry" in
+   none)
+      AC_DEFINE([CHEMISTRY_NONE], [1], [No chemistry function])
+   ;; 
+   grackle)
+      AC_DEFINE([CHEMISTRY_GRACKLE], [1], [Chemistry compatible with the grackle cooling])
+   ;; 
+   *)
+      AC_MSG_ERROR([Unknown chemistry function: $with_chemistry])
+   ;;
+esac
+
 #  External potential
 AC_ARG_WITH([ext-potential],
    [AS_HELP_STRING([--with-ext-potential=<pot>],