pipelight: Clean up PKGBUILD script

A lot of the complicated constructs have historic reasons, and
there is no reason to keep them. This includes:

* Manually adjusting CXXFLAGS to compile static not necessary
  anymore, since the configure switch --win32-static is used.

* Manually adjusting gccRuntimeDlls.. in the configs not required,
  its possible to set --gcc-runtime-dlls="" instead.

* The install-dependency replacement actually never had any effect,
  because pipelight comes with its own install-dependency script
  since some time.

* Manually installing pluginloader.exe is also not required, its
  easier to just let the Makefile do this job. To make this
  possible the pluginloader.tar.gz tarball is extracted inside
  $_srcdir.