Add callgraph ninja target

[hadess]

In a manner similar to the scan-build ninja target, it would be great if it was possible to create a target that output a callgraph as a "dot" file.

The current process goes something like this:

• Set CC="clang" CPPFLAGS="-emit-llvm" and run your build as usual, until clang is unable to link your files with file not recognized: File format not recognized (the generated .o files are LLVM IR bitcode).
• Merge the .o files for your target (a library in this case): llvm-link -o foo *.o
• Generate a callgraph.dot file with that: cat foo | opt -dot-callgraph

And then trying using graphviz' dot to generate a callgraph. Or use cat foo | opt -print-callgraph to print a human-readable output, etc.

[hadess]

I also found cflow, for C files, which is probably slightly easier to use, although I don't know how accurate its C parsing is.

$ cflow <list of files.c>