git: fix PKGLIST logic
git: fix PKGLIST logic

The logic to determine make targets depending on the value of the
`PKGLIST` variable assumed that said variable is *not* an array.
However, it *is* an array.

While at it, avoid building the executables when only packaging
documentation.

Signed-off-by: Johannes Schindelin <53fb8db7833fca1e1746dd8592f61048d350f64c@gmx.de>

JanSchulz/MINGW-packages,JanSchulz/MINGW-packages,vcgato29/MINGW-packages,dscho/MINGW-packages,git-for-windows/MINGW-packages,dscho/MINGW-packages,vcgato29/MINGW-packages,vcgato29/MINGW-packages,dscho/MINGW-packages,git-for-windows/MINGW-packages,dscho/MINGW-packages,git-for-windows/MINGW-packages,dscho/MINGW-packages,JanSchulz/MINGW-packages