chore(tests/*/test.sh): change working dir; avoid using ls in for-loop
chore(CMakeLists.txt): add EMACS_LIB option to specify installation path for emacs stuff
chore(.travis.osx.yml): add configurations using clang++
doc(make/msys2): update instructions
chore(util/*): add cstddef header before including gmp.h/mpfr.h

Reference: https://gcc.gnu.org/gcc-4.9/porting_to.html

------

Header <cstddef> changes

The <cstddef> header was updated for C++11 support and this breaks some
libraries which misuse macros meant for internal use by GCC only. For
instance with GMP versions up to 5.1.3, you may see:

/usr/include/c++/4.9.0/cstddef:51:11: error: ‘::max_align_t’ has not been declared
   using ::max_align_t;
           ^
Another possible error is:

someheader.h:99:13: error: ‘ptrdiff_t’ does not name a type

A workaround until libraries get updated is to include <cstddef> or
<stddef.h> before any headers from that library.
fix(emacs/lean-mode): wrong pp highlighting, fixes #225