hardcoded gmake

I was getting the following error while building on OS X:

```
=====================================================================
=  CREATING DOC DATABASE (mkdoc.sh)
=  Thu Mar 28 18:44:08 MSK 2013
=====================================================================

vspx_doc.sh: line 44: gmake: command not found
```

Build was continuing ignoring the error. This commit helps shell-script to find `make` command