build: Do not build (guix build syscalls) if 'mount' is missing from libc.

This disables compilation of this module on GNU/Hurd.
Reported by Manolis Ragkousis <manolis837@gmail.com>.

* m4/guix.m4 (GUIX_CHECK_LIBC_MOUNT): New variable.
* configure.ac: Use it.  Define 'BUILD_SYSCALLS_MODULE' conditional.
* Makefile.am (MODULES, EXTRA_DIST): Make 'guix/build/syscalls.scm'
  conditional on BUILD_SYSCALLS_MODULE.

Makefile.am

@@ -88,7 +88,6 @@ MODULES =					\
   guix/build/rpath.scm				\
   guix/build/cvs.scm				\
   guix/build/svn.scm				\
-  guix/build/syscalls.scm			\
   guix/build/gremlin.scm			\
   guix/build/emacs-utils.scm			\
   guix/build/graft.scm				\
@@ -124,6 +123,13 @@ MODULES =					\
   guix.scm					\
   $(GNU_SYSTEM_MODULES)
 
+if BUILD_SYSCALLS_MODULE
+
+MODULES +=					\
+  guix/build/syscalls.scm
+
+endif
+
 if BUILD_DAEMON_OFFLOAD
 
 MODULES +=					\
@@ -298,6 +304,13 @@ EXTRA_DIST +=					\
 
 endif !BUILD_DAEMON_OFFLOAD
 
+if !BUILD_SYSCALLS_MODULE
+
+EXTRA_DIST +=					\
+  guix/build/syscalls.scm
+
+endif !BUILD_SYSCALLS_MODULE
+
 
 CLEANFILES =					\
   $(GOBJECTS)					\

configure.ac

@@ -85,6 +85,11 @@ dnl Check whether (srfi srfi-37) works, and provide our own if it doesn't.
 GUIX_CHECK_SRFI_37
 AM_CONDITIONAL([INSTALL_SRFI_37], [test "x$ac_cv_guix_srfi_37_broken" = xyes])
 
+dnl Check whether (guix build syscalls) can be built.
+GUIX_CHECK_LIBC_MOUNT
+AM_CONDITIONAL([BUILD_SYSCALLS_MODULE],
+  [test "x$guix_cv_libc_has_mount" = "xyes"])
+
 AC_ARG_WITH([nix-prefix],
   [AS_HELP_STRING([--with-nix-prefix=DIR],
     [search for Nix in DIR (for testing purposes and '--disable-daemon' builds)])],

m4/guix.m4

@@ -257,3 +257,16 @@ AC_DEFUN([GUIX_ASSERT_CXX11], [
     AC_MSG_ERROR([C++ compiler '$CXX' does not support the C++11 standard])
   fi
 ])
+
+dnl GUIX_CHECK_LIBC_MOUNT
+dnl
+dnl Check whether libc provides 'mount'.  On GNU/Hurd it doesn't (yet).
+AC_DEFUN([GUIX_CHECK_LIBC_MOUNT], [
+  AC_CACHE_CHECK([whether libc provides 'mount'], [guix_cv_libc_has_mount],
+    [GUILE_CHECK([retval], [(dynamic-func \"mount\" (dynamic-link))])
+     if test "$retval" = 0; then
+       guix_cv_libc_has_mount="yes"
+     else
+       guix_cv_libc_has_mount="no"
+     fi])
+])