Speedup CI builds using cotire

[tusharpm]

All unit-*.cpp files in test/src include catch.hpp and json.hpp. Both are header-only libraries, and can benefit from using precompiled headers.

• Use cotire for reducing the compile time using a prefix header (test/src/prefix.hpp).
• Change travis config to use cmake instead of the hand-written Makefiles.
• Enable parallel build in Travis config (-- -j4)

The results - comparing a sample without this change and with:

Metric	Without cotire	With cotire
Travis (Running time)	32 min 21 sec	28 min 32 sec
Travis (Total time)	2 hrs 3 min 43 sec	1 hr 52 min 36 sec
AppVeyor (Total time)	3 min 34 sec	2 min 55 sec

The travis build time can be further reduced by using the cmake build system for the slowest pieces (valgrind and sanitizer - these recompile the tests independently now), and the container-infrastructure.

[coveralls]

Coverage remained the same at 99.964% when pulling 5436407 on tusharpm:cotire into 0200f2d on nlohmann:develop.

[nlohmann]

I have some questions:

• What is the role of file test/src/prefix.hpp?
• To what extend is the speedup related to cotire and where is it just using -j4?

[tusharpm]

• test/src/prefix.hpp is the prefix header (manually maintained, instead of the one auto-generated by cotire) including the headers that need to be precompiled. As per my understanding, this (or a precompiled equivalent anyway) is added to all compilation units by cotire.
• I triggered a build without -- -j4 just for comparing those results as well. Overall, it is a little slower than the original build (no-cotire).
  • The "special" builds are generally-speaking slower (because of the second compilation not using cmake).
  • The remaining (compile-heavy) builds have mixed results.
    • Some are a tad faster (LLVM - 3.8.0 and 3.8.1 saving 15-20 seconds in about 4 minutes, similar for Xcode 8.2).
    • Others are considerably slower (Xcode 6.4 is roughly 2 minutes slower from the original 6½ minutes).
  • I agree this analysis is not statistically sound (very few data points), but it gives a rough idea of the time taken by each variant.
  • The advantage of (only) using cotire is visible on the AppVeyor build which doesn't have an explicit parallel jobs specification.
  • If the explicit -- -j4 is the problem, should I try with a different generator/build-system (ninja), which can determine the best configuration at runtime?

[nlohmann]

I added a branch from this PR, see https://github.com/nlohmann/json/tree/tusharpm-cotire. I shall merge it when Travis/AppVeyor have finished checking it. Thanks a lot!

[tusharpm]

My pleasure! 👍