(autocf21.info.gz) Writing configure

(autocf21.info.gz) Making configure Scripts
(autocf21.info.gz) Invoking autoscan
 
 2.1 Writing `configure.in'
 ==========================
 
 To produce a `configure' script for a software package, create a file
 called `configure.in' that contains invocations of the Autoconf macros
 that test the system features your package needs or can use.  Autoconf
 macros already exist to check for many features; see  Existing
 Tests, for their descriptions.  For most other features, you can use
 Autoconf template macros to produce custom checks; see  Writing
 Tests, for information about them.  For especially tricky or
 specialized features, `configure.in' might need to contain some
 hand-crafted shell commands.  The `autoscan' program can give you a
 good start in writing `configure.in' ( Invoking autoscan, for
 more information).
 
    The order in which `configure.in' calls the Autoconf macros is not
 important, with a few exceptions.  Every `configure.in' must contain a
 call to `AC_INIT' before the checks, and a call to `AC_OUTPUT' at the
 end ( Output).  Additionally, some macros rely on other macros
 having been called first, because they check previously set values of
 some variables to decide what to do.  These macros are noted in the
 individual descriptions ( Existing Tests), and they also warn
 you when creating `configure' if they are called out of order.
 
    To encourage consistency, here is a suggested order for calling the
 Autoconf macros.  Generally speaking, the things near the end of this
 list could depend on things earlier in it.  For example, library
 functions could be affected by typedefs and libraries.
 
      `AC_INIT(FILE)'
      checks for programs
      checks for libraries
      checks for header files
      checks for typedefs
      checks for structures
      checks for compiler characteristics
      checks for library functions
      checks for system services
      `AC_OUTPUT([FILE...])'
 
    It is best to put each macro call on its own line in `configure.in'.
 Most of the macros don't add extra newlines; they rely on the newline
 after the macro call to terminate the commands.  This approach makes
 the generated `configure' script a little easier to read by not
 inserting lots of blank lines.  It is generally safe to set shell
 variables on the same line as a macro call, because the shell allows
 assignments without intervening newlines.
 
    When calling macros that take arguments, there must not be any blank
 space between the macro name and the open parenthesis.  Arguments can be
 more than one line long if they are enclosed within the `m4' quote
 characters `[' and `]'.  If you have a long line such as a list of file
 names, you can generally use a backslash at the end of a line to
 continue it logically on the next line (this is implemented by the
 shell, not by anything special that Autoconf does).
 
    Some macros handle two cases: what to do if the given condition is
 met, and what to do if the condition is not met.  In some places you
 might want to do something if a condition is true but do nothing if it's
 false, or vice versa.  To omit the true case, pass an empty value for
 the ACTION-IF-FOUND argument to the macro.  To omit the false case,
 omit the ACTION-IF-NOT-FOUND argument to the macro, including the comma
 before it.
 
    You can include comments in `configure.in' files by starting them
 with the `m4' builtin macro `dnl', which discards text up through the
 next newline.  These comments do not appear in the generated
 `configure' scripts.  For example, it is helpful to begin
 `configure.in' files with a line like this:
 
      dnl Process this file with autoconf to produce a configure script.
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(autocf21.info.gz) Making configure Scripts
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