Fix bug 7209: Stack overflow on circular enum with explicit type

Detect the error in the semantic pass. This slightly changes the error
message for one existing test (it was previously detected inside CTFE)
-- now the *first* circular definition generates the error, which makes
more sense.
Merge pull request #3594 from donc/circular_enum7209

Fix bug 7209: Stack overflow on circular enum with explicit type
Add manually ported frontend source files to version control
Add settings file and makefile targets to build ddmd
fix posix.mak ddmd rule
Add generated source to .gitignore
Add magicport source to repo
Delete unconvertable comment
Move #defined constants outside class declaration
Update json file