The configure script checks its command-line options against a list of known options, like --help or --config-cache. An unknown option ordinarily indicates a mistake by the user and configure halts with an error. However, by default unknown --with-package and --enable-feature options elicit only a warning, to support configuring entire source trees.
Source trees often contain multiple packages with a top-level
configure script that uses the AC_CONFIG_SUBDIRS macro
(see Subdirectories). Because the packages generally support
different --with-package and
--enable-feature options, the GNU Coding
Standards say they must accept unrecognized options without halting.
Even a warning message is undesirable here, so AC_CONFIG_SUBDIRS
automatically disables the warnings.
This default behavior may be modified in two ways. First, the installer
can invoke configure --disable-option-checking to disable
these warnings, or invoke configure --enable-option-checking=fatal
options to turn them into fatal errors, respectively. Second, the
maintainer can use AC_DISABLE_OPTION_CHECKING.
By default, disable warnings related to any unrecognized --with-package or --enable-feature options. This is implied by
AC_CONFIG_SUBDIRS.The installer can override this behavior by passing --enable-option-checking (enable warnings) or --enable-option-checking=fatal (enable errors) to configure.