enable travis to use IPO and add a test using libc++

[nolange]

I dont know anything about travis, hope this works correctly.

see #18

[nolange]

last commit for me today, if clang + libc++ still doesnt builds, then allow it as failure.

[martinmoene]

I think I get the importance of applying LTO to be better informed about software's correctness. Introducing it might take a considerable amount of time, as I want to know enough about it with respect to CMake, compilers and Travis to reach an easy-to-follow implementation.

[nolange]

I can build your tests with clang + LTO locally (with libc++ on top if d70b03c is applied), thats not the issue here.
CMake has trouble picking up the matching C/C++ Compiler/Linker/Binutils, I think it has some holes in its automatic detection if suffixes and non-standard paths are involved (guess it never could figure out every possible installation)

ie look at the gcc-7 output:

CMAKE_CXX_COMPILER:FILEPATH=/usr/lib/ccache/g++-7
CMAKE_CXX_COMPILER_AR:FILEPATH=/usr/bin/gcc-ar-7
CMAKE_C_COMPILER:FILEPATH=/usr/lib/ccache/gcc
CMAKE_C_COMPILER_AR:FILEPATH=/usr/bin/gcc-ar-4.8

with clang this is even more of a mess. likely you need to tell CMake the full paths to C/C++ compilers and tools (AR at least).

[Flamefire]

Not sure if this fixes your failures but when setting CXX you should also set CC. Suggested approach is to put MATRIX_EVAL="CXX=clang++-6 CC=clang-6" into the env section and then do eval "${MATRIX_EVAL}" in the script. (See the travis docu on C++)