Changes

*/gen_links: remove TCCOpenSecurity_Functions.hh

The file TCCOpenSecurity_Functions.hh does not exist in a fresh clone of
titan.Libraries.TCCUsefulFunctions. Currently it only gets created after
gen_links.sh ran, created the symlink (to the not yet existing file) and
then ttcn3_compiler writes to it. Adjust the gen_links files to not
create this broken symlink.

Related: https://gitlab.eclipse.org/eclipse/titan/titan.Libraries.TCCUsefulFunctions/-/tree/master/src
Change-Id: Idab86fa1deb0256737167767ee7f5f2b36018df4

buildsystem/gen_links: catch broken symlinks

Tweak gen_links() to verify that each symlink target exists. If the
target does not exist, abort with an error:

  ERROR in msc/gen_links.sh:

    File not found:
      ../deps/titan.Libraries.TCCUsefulFunctions/src/TCCOpenSecurity_Functions.hh

    Section:
      DIR=../deps/titan.Libraries.TCCUsefulFunctions/src
      FILES="…TCCOpenSecurity_Functions.hh…"
      gen_links $DIR $FILES

This is slightly slower than the previous approach (which did not check
if the symlink target exists). But this prevents adding subtle bugs
that may not trigger every time (see 520ac3ab ("*/gen_links.sh: fix
wrong SDP_EncDec.cc symlink")).

Change-Id: I0da0ec30e5eaf65d0348e22900a5ee006dee9e44