installed 'gropdf' incorrectly used for compilation

[Werner LEMBERG]

While building current git from groff (2f4432718c) with

```
./bootstrap --gnulib-srcdir=~/git/gnulib
./configure
make -j12
```

I got the following error messages:

```
gropdf:contrib/mom/examples/typesetting.mom: fatal error:
  failed to open '/usr/share/ghostscript/9.52/Resource/Font/C059-Roman'
gropdf:src/utils/addftinfo/addftinfo.1: fatal error:
  failed to open '/usr/share/ghostscript/9.52/Resource/Font/NimbusRoman-Italic'
```

Analyzing the problem showed that I have an older groff installation
in `/usr/local` that was configured for gs 9.52.  However, my
currently installed gs version is 9.54; this means that my locally
installed groff version is outdated and thus partially invalid.

The installation from git gets correctly configured to use 9.54.

This inconsistency of gs on my GNU/Linux is of no importance, but it
clearly shows that there is a bug while compiling groff: It uses
`gropdf` from my system instead of using the just compiled `gropdf`!

I guess that during the documentation build `groff -Tpdf` gets used
without adjusting the PATH so that the freshly compiled binary doesn't
come first.


    Werner