contrib: Add support for out-of-tree builds in gen-manpages.sh

This adds support for setting the environment variable `BUILDDIR`
to point to executables that are outside the source directory.

E.g. to invoke the tool when the build is in $PWD/build:

```bash
BUILDDIR=$PWD/build contrib/devtools/gen-manpages.sh
```

contrib/devtools/README.md

@@ -85,6 +85,14 @@ gen-manpages.sh
 A small script to automatically create manpages in ../../doc/man by running the release binaries with the -help option.
 This requires help2man which can be found at: https://www.gnu.org/software/help2man/
 
+With in-tree builds this tool can be run from any directory within the
+repostitory. To use this tool with out-of-tree builds set `BUILDDIR`. For
+example:
+
+```bash
+BUILDDIR=$PWD/build contrib/devtools/gen-manpages.sh
+```
+
 git-subtree-check.sh
 ====================