Add popen feature

* doc/ref/api-options.texi (Common Feature Symbols): Document the popen
  feature.
* doc/ref/posix.texi (Pipes): Depend on the popen feature, not fork.
* libguile/posix.c (scm_init_posix): Add popen feature if we can.

doc/ref/api-options.texi

@@ -284,8 +284,11 @@ Indicates support for POSIX functions: @code{pipe}, @code{getgroups},
 
 @item fork
 Indicates support for the POSIX @code{fork} function (@pxref{Processes,
-@code{primitive-fork}}).  This is a prerequisite for the @code{(ice-9
-popen)} module (@pxref{Pipes}).
+@code{primitive-fork}}).
+
+@item popen
+Indicates support for @code{open-pipe} in the @code{(ice-9 popen)}
+module (@pxref{Pipes}).
 
 @item random
 Indicates availability of random number generation functions:

doc/ref/posix.texi

@@ -2262,7 +2262,7 @@ controlling terminal.  The return value is unspecified.
 The following procedures are similar to the @code{popen} and
 @code{pclose} system routines.  The code is in a separate ``popen''
 module@footnote{This module is only available on systems where the
-@code{fork} feature is provided (@pxref{Common Feature Symbols}).}:
+@code{popen} feature is provided (@pxref{Common Feature Symbols}).}:
 
 @lisp
 (use-modules (ice-9 popen))

libguile/posix.c

@@ -2339,6 +2339,7 @@ scm_init_posix ()
   scm_add_feature ("fork");
 #endif	/* HAVE_FORK */
 #ifdef HAVE_START_CHILD
+  scm_add_feature ("popen");
   scm_c_register_extension ("libguile-" SCM_EFFECTIVE_VERSION,
                             "scm_init_popen",
 			    (scm_t_extension_init_func) scm_init_popen,