Fix Shellcheck errors
After applying the attached patches, the only error I'm seeing are:
In yelp-build.in line 26: yelp_icon_dir='@YELP_ICON_DIR@' ^-- SC2034: yelp_icon_dir appears unused. Verify it or export it.
In yelp-build.in line 327: if [ "x$minput_rel" != "x$minput_src" ]; then ^-- SC2154: minput_src is referenced but not assigned.
but these need someone who's familiar with the code. Othewise there are no error except of SC2016, but these are sort of false positives (it thinks that the shell variables are used inside string, but these are not shell variables).