Add new sparse matmul performance tracking test

Existing test for sparse matrix-matrix multiplication is not very representative
Tweaks for cross compiling from Cygwin to MinGW

* Cygwin versions of MinGW compilers put standard libraries in /bin, not /lib
* Blas and Lapack should be in build_shlibdir
* Don't use wine for Cygwin to MinGW cross compile
* New cygpath_w helper function to convert Cygwin paths to Windows-style so MinGW-compiled
executables can understand them
* Use cygpath_w for system image file locations
Fix doc example of @test_approx_eq_eps precision argument 

`e-2` means Euler's number minus two. I guess original author wanted to mean `10^-2`
fix #5966

showcompact() should always be compact; confine the output limit stuff to
array printing
Merge pull request #5968 from cdsousa/patch-1

Fix doc example of @test_approx_eq_eps precision argument
fix #5963
Merge pull request #5922 from tkelman/patch-1

Add new sparse matmul performance tracking test
Merge pull request #5965 from tkelman/cygwin-to-mingw-cross

RFC: Allow Cygwin to MinGW cross compilation
Update Brazilian Portuguese documentation link

For reasons similar to 096893a90.
Comment out test for balancing. Failing with LAPACK 3.5, because balancing has been improved.
Merge pull request #5976 from JuliaLang/anj/eigtest

Comment out test for balancing. Failing with LAPACK 3.5, because balancing has been improved.
Merge pull request #5974 from andrioni/patch-4

Update Brazilian Portuguese documentation link