Respect DESTDIR setting when installing config files

I think, it's not very convenient that config files are getting installed to `$(CFGDIR)`, they should go to the `$(DESTDIR)/$(CFGDIR)` instead.
I use the following commands to build cppcheck:
```
make \
 CXXFLAGS="-O2 -mtune=native -pipe -DNDEBUG -Wall" \
 SRCDIR=build \
 PREFIX=/usr \
 CFGDIR=/usr/share/cppcheck/cfg/ \
 HAVE_RULES=yes || exit 1
make install \
 PREFIX=/usr \
 CFGDIR=/usr/share/cppcheck/cfg/ \
 DESTDIR=/tmp/tgz/package-cppcheck || exit 1
```