(automake.info.gz) Conditionals

 
 Conditionals
 ************
 
    Automake supports a simple type of conditionals.
 
    Before using a conditional, you must define it by using
 `AM_CONDITIONAL' in the `configure.in' file ( Macros).
 
  - Macro: AM_CONDITIONAL (CONDITIONAL, CONDITION)
      The conditional name, CONDITIONAL, should be a simple string
      starting with a letter and containing only letters, digits, and
      underscores.  It must be different from `TRUE' and `FALSE' which
      are reserved by Automake.
 
      The shell CONDITION (suitable for use in a shell `if' statement)
      is evaluated when `configure' is run.  Note that you must arrange
      for _every_ `AM_CONDITIONAL' to be invoked every time `configure'
      is run - if `AM_CONDITIONAL' is run conditionally (e.g., in a
      shell `if' statement), then the result will confuse automake.
 
    Conditionals typically depend upon options which the user provides to
 the `configure' script.  Here is an example of how to write a
 conditional which is true if the user uses the `--enable-debug' option.
 
      AC_ARG_ENABLE(debug,
      [  --enable-debug    Turn on debugging],
      [case "${enableval}" in
        yes) debug=true ;;
        no)  debug=false ;;
        *) AC_MSG_ERROR(bad value ${enableval} for --enable-debug) ;;
      esac],[debug=false])
      AM_CONDITIONAL(DEBUG, test x$debug = xtrue)
 
    Here is an example of how to use that conditional in `Makefile.am':
 
      if DEBUG
      DBG = debug
      else
      DBG =
      endif
      noinst_PROGRAMS = $(DBG)
 
    This trivial example could also be handled using EXTRA_PROGRAMS
 ( Conditional Programs).
 
    You may only test a single variable in an `if' statement, possibly
 negated using `!'.  The `else' statement may be omitted.  Conditionals
 may be nested to any depth.  You may specify an argument to `else' in
 which case it must be the negation of the condition used for the
 current `if'.  Similarly you may specify the condition which is closed
 by an `end':
 
      if DEBUG
      DBG = debug
      else !DEBUG
      DBG =
      endif !DEBUG
 
 Unbalanced conditions are errors.
 
    Note that conditionals in Automake are not the same as conditionals
 in GNU Make.  Automake conditionals are checked at configure time by the
 `configure' script, and affect the translation from `Makefile.in' to
 `Makefile'.  They are based on options passed to `configure' and on
 results that `configure' has discovered about the host system.  GNU
 Make conditionals are checked at `make' time, and are based on
 variables passed to the make program or defined in the `Makefile'.
 
    Automake conditionals will work with any make program.
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