Immediate assignment, datetime to snapshots builds

Immediate assignment avoids multiple evaluation and removes potential to
race with e.g. build timestamps.  The changes made here don't really have
an impact because e.g. GIT_DESCRIBE won't change during a build.
Improve git describe options: --always, --dirty

--dirty is important to indicate a build with uncommitted changes.
Merge branch 'makefile-improvements'

Use immediate assignment to avoid (quite theoretical) races when multiple
build targets re-evalute things like "git describe".  Add build timestamp
to snapshot builds (this *does* have a race without immediate assignment).

Improve git describe options to mark dirty snapshot builds.
Ditz issue update
Add more stuff to .gitignore
Merge branch 'gitignore-additions'

Some .gitignore fixes.
Add Object.assign polyfill

More or less untested.
Add Performance.now() polyfill

More or less untested.
Add RegExp.prototype.compile() polyfill
Polyfill comment updates
Add polyfills to dist

Leave RegExp.prototype.compile() out as non-working.
Release note: more polyfills
Merge branch 'more-polyfills'

Add some more polyfills: Object.assign() and Performance.now().

The RegExp.prototype.compile() polyfill cannot be implemented at the moment
because it needs to overwrite some RegExp instance properties which are
currently protected.
.gitignore: dist/ and autotest/
Emscripten fmod() test
Merge branch 'misc-repo-cleanups'
Ditz issue update
Clarify no-zeroing of duk_alloc() & co in docs
Remove mem zeroing test from memory func API test

The memory allocation is not zeroed automatically.
Merge branch 'clarify-duk-alloc-zeroing'

The memory functions duk_alloc(), duk_relloc(), and their _raw()
counterparts don't zero allocated (reallocated) memory.  This is
intentional, as it also matches the behavior of standard malloc()
and friends.

Clarify this behavior in API documentation, and fix one API testcase
which was testing for memzeroing behavior.