Fix APXS error "cannot determine bootstrap symbol name"

Fix APXS error "cannot determine bootstrap symbol name"

Maybe related to out-of-source builds?

```
test -d /target/usr/lib/apache2/modules || mkdir -p /target/usr/lib/apache2/modules
/usr/bin/apxs2 -i -S LIBEXECDIR=/target/usr/lib/apache2/modules mod_auth_gssapi.la
apxs:Error: Sorry, cannot determine bootstrap symbol name.
apxs:Error: Please specify one with option `-n'.
Makefile:725: recipe for target 'install-exec-local' failed
```

Reviewed-by: Simo Sorce <simo@redhat.com>
Close #79