Calling AC_CANONICAL_SYSTEM implies AC_CANONICAL_TARGET, which
gives incorrect results when cross compiling. Calling only
AC_CANONICAL_HOST is enough.

issue: 129
Install jpwl man pages only if jpwl is enabled

issue: 129
Improve check for standard libs

Previously, with <lib>_LIBS and <lib>_CFLAGS undefined,
the most basic compile command "$CC conftest.c -l<lib>" was not
tested.

issue: 129