autogen: check for needed commands in autogen.sh

And display a more informative message.
autogen: check if pkg-config is correctly installed

Absence of pkg-config leads to some odd messages:

    configure.ac:27: error: possibly undefined macro: AC_SUBST
          If this token and others are legitimate, please use m4_pattern_allow.
          See the Autoconf documentation.
    configure.ac:163: error: possibly undefined macro: AC_MSG_FAILURE
    configure:21413: error: possibly undefined macro: AC_MSG_CHECKING
    configure:21418: error: possibly undefined macro: AC_TRY_LINK
    configure:21423: error: possibly undefined macro: AC_MSG_RESULT
    configure:21432: error: possibly undefined macro: AC_MSG_NOTICE
    configure:21894: error: possibly undefined macro: AC_PATH_TOOL
    configure:21896: error: possibly undefined macro: AC_MSG_ERROR
    configure:21975: error: possibly undefined macro: AC_DEFINE_UNQUOTED

We try to detect this situation by looking for `PKG_CHECK_MODULES` macro
in `aclocal.m4`. If not present, we issue some useful tips, like
installing the package or telling aclocal the location of
`pkg.m4` (mostly for OSX users).