CMake: Skip empty link.txt lines (#13845)

In the implementation of "cmake -E cmake_link_script", skip lines from
the input file that are empty or contain only whitespace.  Do not try to
run a child with no command line.
KWSys 2013-01-10 (608d6b47)

Extract upstream KWSys using the following shell commands.

$ git archive --prefix=upstream-kwsys/ 608d6b47 | tar x
$ git shortlog --no-merges --abbrev=8 --format='%h %s' fc60c8b8..608d6b47
Rolf Eike Beer (6):
      297758a5 SystemInformation: fix conversion warning
      79ef34ef SystemInformation: fix calling kwsysProcess_WaitForData()
      f1068caf SystemInformation: speed up copying process data
      7dfc27d5 SystemInformation: check CPU vendor and SSE support on OpenBSD
      494d9d7a SystemInformation: get stepping code on Intel Macs
      608d6b47 SystemInformation: determine processor features on Intel Macs

Change-Id: I7f5bc5b7af2bf7d4e5c1ee291c286add0f17a7d5
Merge branch 'upstream-kwsys' into update-kwsys
Don't include generator expressions in old-style link handling.

Don't add generator expressions to variables which are used
for CMP0003, CMP0004, and the old-style _LIB_DEPENDS content. They
will not be evaluated when read anyway and would probably confuse
the code reading them.

This makes it legitimate to use target_link_libraries with generator
expressions as arguments.
Document the use of generator expressions in new commands.
Add the TARGET_DEFINED generator expression

This tests whether the parameter is a usable target.
KWSys 2013-01-14 (6fa1c99f)

Extract upstream KWSys using the following shell commands.

$ git archive --prefix=upstream-kwsys/ 6fa1c99f | tar x
$ git shortlog --no-merges --abbrev=8 --format='%h %s' 608d6b47..6fa1c99f
Brad King (1):
      84827cc1 Process: Avoid argv[0]==NULL from parsing empty command line

Rolf Eike Beer (8):
      e041cd66 SystemInformation: factor out QueryMemoryBySysconf()
      ea850fa0 SystemInformation: factor out QueryProcessorBySysconf()
      57f06d49 SystemInformation: count processors with sysconf() on Solaris
      c6057a06 SystemInformation: fix "Multi-character character constant"
      5a27bd4f SystemInformation: make IsHyperThreadingSupported() return bool
      342c0ad2 SystemInformation: query memory size with sysconf() on Solaris
      f0b857c1 SystemInformation: cache result of IsHyperThreadingSupported()
      ab0c2a09 SystemInformation: try using assembler with BorlandC

Change-Id: I072371ed35eed892a5ef62a9e9e6cad734e961d9
Merge branch 'upstream-kwsys' into update-kwsys
Add a way to check INTERFACE user property compatibility.
Merge topic 'fix-empty-link-line-crash'

8ff1d47 CMake: Skip empty link.txt lines (#13845)
Merge topic 'compatible-INTERFACE-user-properties'

7171fd0 Add a way to check INTERFACE user property compatibility.
Merge topic 'tll-allow-genex'

1da7502 Don't include generator expressions in old-style link handling.
Merge topic 'commands-genex-docs'

7c7b94f Document the use of generator expressions in new commands.
Merge topic 'TARGET_DEFINED-genex'

2bee6f5 Add the TARGET_DEFINED generator expression
Merge topic 'update-kwsys'

e2c756f Merge branch 'upstream-kwsys' into update-kwsys
4ba0ac7 KWSys 2013-01-14 (6fa1c99f)
6f57a90 Merge branch 'upstream-kwsys' into update-kwsys
6318834 KWSys 2013-01-10 (608d6b47)