Raphael Kubo da Costa authored Sep 15, 2015 and Brad King committed Sep 15, 2015

Support for C11's _Thread_local was introduced in GCC in the 4.9 series,
even though we make the C11 compiler flags available in CMake with GCC
>= 4.6.

FreeBSD's runetype.h uses _Thread_local, which causes CMake's own build
to fail when using GCC < 4.9 and -std=gnu11:

  /usr/include/runetype.h:92:22: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'const'
   extern _Thread_local const _RuneLocale *_ThreadRuneLocale;

Add a test for _Thread_local support and only build CMake itself with
C11 support if it works.

Bug: http://www.cmake.org/Bug/view.php?id=15741

Since commit 183b9509 (Follow all dependencies of shared library private
dependencies, 2011-12-14) we honor LINK_INTERFACE_LIBRARIES when
following dependent shared libraries.  The link interface properties may
form a cycle if set incorrectly by a project.  Furthermore, the property
LINK_DEPENDENT_LIBRARIES may form a cycle if set incorrectly by hand
(though CMake should never generate one).  In either case, do not follow
the cycle forever when following the dependent shared library closure.
We only need to add dependency edges to the constraint graph once.

Add "LinkInterfaceLoop" test to cover this case.

Add a LinkFlags test series to check that these properties work.  Since
no link flag is accepted everywhere we test for presence of flags by
adding a bad flag and looking for the complaint in the test output.