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14.3 Choosing Package Options

If a software package has optional compile-time features, the user can give configure command line options to specify whether to compile them. The options have one of these forms: 

     --enable-feature[=arg]
     --disable-feature

These options allow users to choose which optional features to build and install. --enable-feature options should never make a feature behave differently or cause one feature to replace another. They should only cause parts of the program to be built rather than left out.

The user can give an argument by following the feature name with ‘=’ and the argument. Giving an argument of ‘no’ requests that the feature not be made available. A feature with an argument looks like --enable-debug=stabs. If no argument is given, it defaults to ‘yes’. --disable-feature is equivalent to --enable-feature=no.

configure scripts do not complain about --enable-feature options that they do not support. This behavior permits configuring a source tree containing multiple packages with a top-level configure script when the packages support different options, without spurious error messages about options that some of the packages support. An unfortunate side effect is that option spelling errors are not diagnosed. No better approach to this problem has been suggested so far.

For each optional feature, configure.ac should call AC_ARG_ENABLE to detect whether the configure user asked to include it. Whether each feature is included or not by default, and which arguments are valid, is up to you.

— Macro: AC_ARG_ENABLE (feature, help-string, [action-if-given], [action-if-not-given])

If the user gave configure the option --enable-feature or --disable-feature, run shell commands action-if-given. If neither option was given, run shell commands action-if-not-given. The name feature indicates an optional user-level facility. It should consist only of alphanumeric characters and dashes.

The option's argument is available to the shell commands action-if-given in the shell variable enableval, which is actually just the value of the shell variable enable_feature, with any - characters changed into ‘_’. You may use that variable instead, if you wish. The help-string argument is like that of AC_ARG_WITH (see External Software).

You should format your help-string with the macro AS_HELP_STRING (see Pretty Help Strings).

See the examples suggested with the definition of AC_ARG_WITH (see External Software) to get an idea of possible applications of AC_ARG_ENABLE.

— Macro: AC_ENABLE (feature, action-if-given, [action-if-not-given])

This is an obsolete version of AC_ARG_ENABLE that does not support providing a help string.