Make it more explicit and visible when pkg-config is not found

Make it more explicit and visible when pkg-config is not found

Users seem to struggle to read the full error message. This adds an
indication if pkg-config is actually found to the warning/error message
that use pkg-config.

On platforms that do not require pkg-config and for optional libraries,
the existence of pkg-config is mentioned as part of the error/warning message.

When found:

    configure: error: libnl-genl-3.0 package not found or too old. Is the development package and pkg-config (/usr/bin/pkg-config) installed? Must be version 3.4.0 or newer for DCO

not found:

    configure: error: libnl-genl-3.0 package not found or too old. Is the development package and pkg-config (not found) installed? Must be version 3.4.0 or newer for DCO

On platforms where pkg-config is required (only Linux at the moment),
configure will abort when not detecting pkg-config:

checking for pkg-config... no
configure: error: pkg-config is required

Change-Id: Iebaa35a23e217a4cd7739af229cbfc08a3d8854a
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Antonio Quartulli <a@unstable.cc>
Message-Id: <20240105140540.14757-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg27939.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit c0f38019b4a2044c1fe873d7d33c13ce571d3386)