Add --with-gdbscripts-dir=PATH configure option

Add --with-gdbscripts-dir=PATH configure option

Currently the gdb valgrind scripts are installed under VG_LIBDIR
which is normally pkglibexecdir which is likely not in the default
gdb safe-path (a list of directories from which it is safe to
auto-load files). So users will have to add the directory to their
.gdbinit file.

This patch adds a --with-gdbscripts-dir=PATH configure option that
sets VG_GDBSCRIPTS_DIR to the given PATH (${libexecdir}/valgrind if
not given). A user can also configure --without-gdbscripts-dir to
disable adding a .debug_gdb_scripts section to the vgpreload library
and installing the valgrind-monitor python scripts completely.

Use VG_GDBSCRIPTS_DIR as gdbscriptsdir to install the valgrind-monitor
python files and pass it with CPPFLAGS when building vg_preloaded.c
and vgdb.c to use instead of VG_LIBDIR.

https://bugs.kde.org/show_bug.cgi?id=469768