Disable LLVM bitcode generation with pgxs.mk framework.

Hi hackers,

When the PostgreSQL server is configured with --with-llvm, the pgxs.mk
framework will generate LLVM bitcode for extensions automatically.
Sometimes, I don't want to generate bitcode for some extensions. I can
turn off this feature by specifying with_llvm=0 in the make command.

```
make with_llvm=0
```

Would it be possible to add a new switch in the pgxs.mk framework to
allow users to disable this feature? E.g., the Makefile looks like:

```
WITH_LLVM=no
PG_CONFIG = pg_config
PGXS := $(shell $(PG_CONFIG) --pgxs)
```

Best Regards,
Xing