Include compiler switches in dependency checking.

Doesn't include search path because that's irrelevant,
since the actual include files are listed.

Doesn't include macros but should.

Doesn't include switches like -fPIC that distinguish dynamic
from static compilation because the depchecker doesn't know
which model of compiling is used. This is probably a bad bug.

Doesn't include path of the compiler. Probably pointless anyhow
due to use of links when upgrading.

Doesn't include system header file paths. Easy to fix.
Probably should, but would slow down checking.