fix ‘make syntax-check’ complaints (only affects comments).

doc/autoconf.texi

@@ -2049,7 +2049,7 @@ The following macros help you manage the contents of your source tree.
 @defmac AC_CONFIG_SRCDIR (@var{unique-file-in-source-dir})
 @acindex{CONFIG_SRCDIR}
 Distinguish this package's source directory from other source
-directories that might happen to exist in the filesystem.
+directories that might happen to exist in the file system.
 @var{unique-file-in-source-dir} should name a file that is unique to
 this package.  @command{configure} will verify that this file exists in
 @file{@var{srcdir}}, before it runs any other checks.

lib/autoconf/general.m4

@@ -1162,7 +1162,7 @@ if test "$ac_init_help" = "recursive"; then
     _AC_SRCDIRS(["$ac_dir"])
     cd "$ac_dir" || { ac_status=$?; continue; }
     # Check for configure.gnu first; this name is used for a wrapper for
-    # Metaconfig's "Configure" on case-insensitive filesystems.
+    # Metaconfig's "Configure" on case-insensitive file systems.
     if test -f "$ac_srcdir/configure.gnu"; then
       echo &&
       $SHELL "$ac_srcdir/configure.gnu" --help=recursive

lib/autoconf/libs.m4

@@ -312,7 +312,7 @@ AC_LINK_IFELSE([AC_LANG_PROGRAM([@%:@include <X11/Xlib.h>],
   ac_x_includes=
   ac_x_libraries=])
 LIBS="$ac_save_LIBS"
-# If that didn't work, only try xmkmf and filesystem searches
+# If that didn't work, only try xmkmf and file system searches
 # for native compilation.
 AS_IF([test x"$ac_x_includes" = xno && test "$cross_compiling" = no],
   [_AC_PATH_X_XMKMF

lib/autoconf/status.m4

@@ -1191,7 +1191,7 @@ if test "$no_recursion" != yes; then
     cd "$ac_dir"
 
     # Check for configure.gnu first; this name is used for a wrapper for
-    # Metaconfig's "Configure" on case-insensitive filesystems.
+    # Metaconfig's "Configure" on case-insensitive file systems.
     if test -f "$ac_srcdir/configure.gnu"; then
       ac_sub_configure=$ac_srcdir/configure.gnu
     elif test -f "$ac_srcdir/configure"; then