Yesterday I decided that it would have been cool if GNU Smalltalk was compiled as a shared library. With the support for ELF visibilities that is in gcc 3.3, that should have been possible without much performance degradation (note that the installed VM still is linked statically though). Then I also decided that I should compile the shared library with -fomit-frame-pointer because I do need the register that is lost for the GOT pointer when using position-independent code.
The only sane way that came to mind to supply that flag only for the PIC case, and then only for a particular library, was to use libtool 1.5's tags: they are born to support multiple languages, but the presence of the disable-shared tag suggested that they could be put to such use. To summarize, there is absolutely no documentation on how to use tags, not to mention defining new ones (I wanted my tag to be based on the standard tag for C of course, without duplicating all the code in libtool.m4). I have lost a whole afternoon trying to do this, and now that I finally succeeded, what I came up with is a bunch of awful-looking m4 hacks (that luckily can be encapsulated in a separate macro) and with a 770kb configure that is basically doing exactly the same tests twice!
Ah, and I was forgetting to say that it used to top the megabyte until I found out that it was including in the configure file the stuff for C++, Java, and Fortran 77 without ever executing it. :-)