Another bump, posting in case anyone else encounters the same problem and finds this thread (I encountered it while attempting to build pd 0.50 on UbuntuStudio and also a Raspberry Pi).

It's necessary to install the libasound2-dev package as a prerequisite in order to build pd, and the output of 'configure' will show ALSA in both the audio and midi APIs. However, there is another dependency as well; the libavdevice-dev package is also required for the pd executable to support ALSA.

If you've already tried to build pd without this package present, it may be necessary to run:
make clean
./configure
make
This should give a pd executable with working ALSA support.