Changing configure flags for pie build does not result in a full rebuild

[TimWolla]

Working with #83:

I'm not sure if my Makefile / configure script is incorrectly written and does not properly handle the build dependencies, but given the fact that pie always runs both phpize and ./configure for each build, it would probably make sense to run the equivalent of git clean -fdx in the build directory to ensure that the build is internally consistent. Even if the build infrastructure is actually faulty, this could otherwise lead to hard-to-debug issues.

[asgrim]

Yep this is a reasonable improvement we could make - I do think we should clean first. This would only be observable if you are rebuilding an already downloaded version (e.g. pie build a/a:1.2.3 then pie build a/a:1.2.3 --some-flag); otherwise Composer will rip out the source and replace it anyway.