Add optional target suffix for the binary / installation folder.

This is mainly intended to be used for building snapshot binaries that
are able to install in parallel to a release build. E.g. by using the
parameter "SUFFIX=-nightly" for qmake, the target binary will be called
openscad-nightly and the resource base folder /usr/share/openscad-nightly.
Merge pull request #1053 from openscad/install-suffix

Add optional target suffix for the binary / installation folder.