When building for X86 the CMake check_library_exists always fails to find
functions from the Win32 API due to name mangling conventions. The convention
for API functions is __stdcall and the CMake test code assumes __cdecl. Since
these are libraries from the Windows API we can simply link against the
libraries without checking for the functions.

Source: https://github.com/mpeterv/luacheck
Docs:   http://luacheck.readthedocs.org/en/0.12.0/index.html

Run via "make testlint".

I don't want anyone getting the idea that the `-X` flag they might have
used has anything to do with why the `+` is working for them

Before this change, building Neovim would recursively search parent
directories for a .git directory. If Neovim was downloaded as a tarball
(i.e. without a .git directory), but placed in a subdirectory of
a Git repository, this caused a CMake error. Such a situation could
occur when packaging Neovim, for example.

Unfortunately, the previous attempt in #3317 did not fix this problem.

Fixes #3174

 * Split build steps to utilize the Travis build lifecycle.
 * Move shell code from `.travis.yml` into Bash files in `.ci/`,
   one file for each step of the Travis build lifecycle.
 * Use configuration variables in `.travis.yml` to change
   build behavior (e.g. build 32-bit with `BUILD_32BIT=ON`).
 * Keep all configuration in environment variables in
   `.travis.yml`. In scripts, concatenate environment variables
   according to configuration to change to different behavior.
 * Add GCC 5 builds for Linux.
 * Use Travis's caching feature [1] for third-party dependencies
   and pip packages.
 * Allow failures MSan, as the errors it reports have to be
   fixed first.

Valgrind is still disabled, but can be enabled by setting
`env: VALGRIND=ON` for a job in `.travis.yml`.

[1] http://docs.travis-ci.com/user/caching

It unnecessarily complicates spotting linter errors, as they're usually
surrounded by a bunch of lines saying "Done processing ... ".