Add a USE_SYSTEM_EGL define to allow compilation on Unix systems
without X11.

Feature detect __popcnt using Microsoft-specific _MSC_VER rather than
_M_X64/_M_IX86, which can be set by the Dinkumware stdlib on
non-Microsoft systems.

Apple's clang is too old to have -Wextra-semi-stmt

Ran generate_parser.sh using Cygwin64 bison 3.0.4, flex 2.6.4

Bug: 3439
Change-Id: Ie0d01a112a17f70ef60c120063a958b8f1a094f4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1759135
Commit-Queue: James Darpinian <jdarpinian@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>