makefile for metapost: how to compile file-N.pdf

Question

I want to let makefile manage the compilation of figures with metapost. The source file is file.mp. This generates .mps files file.1, file.2 etc. that are then converted to file-1.pdf, file-2.pdf etc.

Here are my rules:

export TEX = latex

%: %.1
        mptopdf $*

%.1: %.mp
        mpost $*

So that when I run make file it creates all the files.

However, I am not satisfied with this solution. Namely, I'd like to be able to let only one of the files be compiled (say file-2.pdf) by entering make file-2. Unfortunately, I don't know how to write the rule for this, although I suspect it might be trivial.

I thought the problem could be solved by extracting the number from the file name given in the command line (i.e. extract 2 from file-2) but it is not clear to me how to do it either.