Only use LTO when compiling Ceres itself, not tests or examples.

- If Ceres is built as a shared library, and LTO is enabled for Ceres
  and the tests, then type_info::operator==() incorrectly returns false
  in gtests' CheckedDowncastToActualType() in the following tests:
-- levenberg_marquardt_strategy_test.
-- gradient_checking_cost_function_test.
  on at least Xcode 6 & 7 as reported here:
  https://github.com/google/googletest/issues/595.
- This does not appear to be a gtest issue, but is perhaps an LLVM bug
  or an RTTI shared library issue.  Either way, disabling the use of
  LTO when compiling the test application resolves the issue.
- Allow LTO to be enabled for GCC, if it is supported.
- Add CMake function to allow easy appending to target properties s/t
  Ceres library-specific compile flags can be iteratively constructed.

Change-Id: I923e6aae4f7cefa098cf32b2f8fc19389e7918c9