build(meson): much more robust build system

build-config/sundials/meson.build

@@ -7,3 +7,11 @@ sundials_dep = declare_dependency(
         kinsol_dep,
     ],
 )
+
+# Vendor the SUNDIALS public headers (see vendor/meson.build). GridFire's
+# installed public headers #include <sundials/...> etc., so consumers of
+# gridfire.pc must compile against exactly the headers this build used --
+# never whatever SUNDIALS happens to be in /usr/local/include (which may be
+# a different version or configuration, e.g. an MPI-enabled build whose
+# sundials_config.h pulls in <mpi.h>).
+subdir('vendor')