Build libgit2 as a dependency, for now requiring CMake
Add libgit2 to the default build
Add OPTS and start tracking libgit2's HEAD
Bump libgit2 to 0.21.0
Fix the Makefile for Linux

Linux uses puts the version at the end of the soname, while OS X puts
it in the middle. Also, case issues.
`make install` now correctly installs libgit2
fixes for Windows cross-compile
add quotes around $(CC) for libgit

also move libgit2 license link to the right section,
and remove command-line git from Versions.make
only send base compiler name to cmake
add a stub test to check libgit2 library version
add cmake and libgit2 info to README.md
also add cmake instructions to README.windows.md

remove notes about issues that should be fixed
Setting OPENBLAS_TARGET_ARCH should disable OPENBLAS_DYNAMIC_ARCH by default
bump libgit2 to v0.21.2
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.
In \, only estimate rank and use complete orthogonal factorization for pivoted qr factorizations.
Merge pull request #8860 from JuliaLang/sf/openblas_dynamic_arch

Setting OPENBLAS_TARGET_ARCH disables OPENBLAS_DYNAMIC_ARCH by default
Small integer powers are no longer slow.

This has been fixed in issue #2741.
Merge pull request #8878 from ntessore/nt/doc-perf

Updated manual: Small integer powers are no longer slow.
Adapt to LLVM3.6 API change