BUG: Ensure only one version of itcl library is built and packaged

Following the update of the tcl version in r23613 (COMP: Update version
of tcl/tk/incrTcl used on Linux/MacOSX. Issue 0003809), it turns out that
the associated source package contains the source of itcl 4.0.0.

Since the external project External_incrTcl also install the version 4.0.1,
duplicate version are built.

This commit explicitly disables all packages available in the source
archive of tcl by deleting the "pkgs" directory. (see below of a complete
list).

The list of disabled packages:
sqlite3.8.0
tdbcmysql1.0.0
tdbcpostgres1.0.0
thread2.7.0
itcl4.0.0
tdbc1.0.0
tdbcodbc1.0.0
tdbcsqlite3-1.0.0

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24468 3bd1e089-480b-0410-8dfb-8563597acbee