Additional cmake tweaks for cygwin

Summary: Fixes issues where
embed.rc does not exist (mark it as generated)
debug mode overflows pe/coff section limitations
an issue with cygwin, folly, and the way gcc + windows headers interact (adding an "internal" define makes headers behave)
force zend compat to not to work/turn on for cygwin

Reviewed By: @ptarjan

Differential Revision: D1455421