Parameter description should be non-empty during build step autodetection

[unix-junkie]

Currently, the auto-detected GNU Autotools build step provides no parameter description:

Instead, the description should read like this:

• configure.ac under /
• configure and configure.ac under /
• configure.ac under / and libxyz/
• configure.ac under / and configure.in under libxyz/

Related: #17

[unix-junkie]

b463d94 .. 65252fb only partially address this issue, as the runner parameters are only properly described if configure.ac and friends are located at the root of the source code tree (i. e. libxyz/configure.ac will be detected but won't be described).