listing all sources

[Trevor Woerner]

I have a project with source files in multiple subdirectories: src,
tests, cfg. Doxygen will generate code documentation based on special
hints in the comments of the code files themselves (like javadoc, if
you're familiar with that mechanism).

I could just blindly have Doxygen run each time I run "make [<something>]":
all-local:: doxygen/html/index.html

But it would be nice if it only ran as required, in other words, only
when one of the project's sources is modified. Previously I had all
the sources in one directory (src) so this was easy:
doxygen/html/index.html: $(SOURCES)
    $(DOXYGEN) Doxyfile

But if the sources are spread throughout the project is there any way
at any one location to dynamically know all the sources?
doxygen/html/index.html: $(SUBDIRS)/$(SOURCES)
    $(DOXYGEN) Doxyfile