configure: fix typo and use PKG_VAR_CHECK.
JFTR for review purposes
- PKG_CHECK_MODULES and PKG_CHECK_VARS are redundant, you need to call just PKG_CHECK_VARS
- PKG_CHECK_VARS have 5 arguments, you are missing checked variable name - due to this autoreconf ended with cryptic error message (configure.ac:35: error: possibly undefined macro: AC_SUBST), that's because the output was too garbled, since the "variable" ($3) argument used was in fact "AC_SUBST([BASH_COMPLETION_DIR], $bash_completion_dir)" and this is too much for m4 to handle
- m4/pkg.m4 was not added to the git repository (I added the slimmed down variant)