[Suggestion] Command line option: List defined version identifiers

[Stewart Gordon]

It would be useful to have a command line option that displays the 
version identifiers that are set.  Maybe -versionlist or something.  For 
example, for DMD on Win32:

C:\>dmd -versionlist
D_InlineAsm
D_InlineAsm_X86
DigitalMars
LittleEndian
X86
Windows
Win32
all

If nothing else is on the command line, it would just print this and 
then exit.  If any other command line options are set, then the list 
will be adjusted accordingly.  If the name of a D file has also been 
given, it would parse the file and output the full list of versions that 
are set for the module as a result of predefined versions, -version 
switches, other command line switches and version assignments in the 
code file.

Having it in DMD might encourage developers of other D compilers 
(particularly those that mimic DMD's CLUI, but potentially any) to 
implement such a facility; and then when this happens, it will become 
even more useful.  It would provide an easy way to:

- determine the name by which a compiler vendor identifies itself
- determine which optional D features a given implementation supports
- check which version blocks will be tested - may be useful for testing 
the portability of code
- debug version manipulation within a module

Stewart.

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