Changes

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