merge the two versions of randmtzig_{randn,exprnd}

It seems to improve slightly (by about 5%) randn() methods working with the
global RNG, which now match the speed of those working with a (non-global)
MersenneTwister instance.
check for and error early with cygwin or msys-target compilers

these are incapable of building libuv, but can build everything else
Document Cygwin-to-MinGW cross compilation

Replace old-and-busted MSYS1 instructions, no one should be using MSYS1 any more.
add a call to env in the git troubleshooting alias
set XC_HOST to `uname -m`-w64-mingw32 by default for cygwin builds
Extend call methods with rounding to Real arguments
Merge branch 'master' into rf/randn

Conflicts:
	base/random.jl
Merge branch 'rf/randn'
Fix building with USECCACHE=1.

This fixes the variable expansion and building with CMake.
Merge pull request #8906 from maleadt/pr_ccache

Fix building with USECCACHE=1.
Merge pull request #8902 from JuliaLang/tk/check-for-cygwin-compilers

RFC: Document Cygwin-to-MinGW cross compilation
Merge pull request #8916 from simonbyrne/convert-rounding2

Extend call methods with rounding to Real arguments
remove trailing whitespace introduced by #8875
Give a better trailing whitespace error message on Travis

Show offending source files with line numbers
escape space-after-colon so yaml parser stops complaining
Merge remote-tracking branch 'upstream/master'