Use relative path to the script, to please python3 on Windows

Use relative path to the script, to please python3 on Windows

Running 'make vs-ide-integration' fails since some recent change
(no idea if that was a cygwin upgrade, or some change in core).
Using 'verbose=t', the last lines are

  make[1]: Leaving directory 'C:/lo/src/build'
  cd C:/lo/src/core &&  C:/lo/src/core/bin/gbuild-to-ide --ide vs --make C:/cygwin64/opt/lo/bin/make
  python3: can't open file '/cygdrive/c/lo/src/core/C:/lo/src/core/bin/gbuild-to-ide': [Errno 2] No such file or directory
  make: *** [Makefile:495: vs-ide-integration] Error 2

It looks like python3 gets called by shell with the script path as
given in the comand line, according to shebang in the script; but
cygwin's python3 can't parse Windows-style path to the script, and
treats it as relative path from cygwin's current directory.

Just use true relative path to the script, to avoid the problem. We
cd to $(SRCDIR) anyway.

Change-Id: Idc25cc74f7a23f0e5733f0d10044986888b43ba1
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/129930
Reviewed-by: Ilmari Lauhakangas <ilmari.lauhakangas@libreoffice.org>
Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>
Tested-by: Jenkins
(cherry picked from commit 18742c27af1c67d41831b5d2297746c299ace3d7)
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/141925
Tested-by: Jenkins CollaboraOffice <jenkinscollaboraoffice@gmail.com>
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>