configure: provide advice if PKG_PROG_PKG_CONFIG unavailable

configure: provide advice if PKG_PROG_PKG_CONFIG unavailable

These are the misleading error messages that this change prevents:
configure.ac:13: error: possibly undefined macro: AC_MSG_WARN
  If this token and others are legitimate, please use m4_pattern_allow.
  See the Autoconf documentation.
configure.ac:48: error: possibly undefined macro: AC_DEFINE
configure.ac:342: error: possibly undefined macro: AC_MSG_ERROR
configure.ac:608: error: possibly undefined macro: AC_MSG_NOTICE

Change-Id: I21bcc7715eeac0f3d0bcc60bba6801e6a895cdd0
Signed-off-by: R. Diez <rdiezmail-openocd@yahoo.de>
Reviewed-on: https://review.openocd.org/c/openocd/+/7035
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>