[Cook] c_incl and -D

[Jason Evans]

It looks like c_incl constructs header dependencies by chasing all #include
directives, but it needn't be so.  If I remember correctly, "gcc -MM" only
generates dependencies for headers that are actually included, given the
current cpp definitions.  As such, -D flags are important in that case.

It doesn't really bother me that c_incl chases all #include directives, but
what is causing me trouble is that c_incl doesn't ignore -D command line
flags, which requires extra complexity in build systems to keep -I and -D
cpp flags separate.  Does it seem reasonable to Peter and others to add
this pseudo feature to c_incl?

Thanks,
Jason