Allow to work on systems without Fcntl::flock implementation.

[Ralf Wildenhues]

This allows to configure git Autoconf on DJGPP, which doesn't provide
file locking.

Note the workaround isn't checking whether flock works on this
particular file system -- the XFile.pm code should already treat that
error condition correctly.  DJGPP perl just barfs when it sees flock,
the code never returns.

The patch is a bit of a hack, as it would be cleaner to put this right
into XFile.pm.  However, that would require us to either fork from the
Automake copy of the file, or also integrate this there.  WDYT?

(For a subsequent 'make' to work, it needs disabling frozen files;
TBD in a later patch.)

Thanks,
Ralf

   Allow to work on systems without Fcntl::flock implementation.
   
   * configure.ac (PERL_FLOCK): New substitution variable with test
   whether Fcntl::flock is implemented by the system.
   * bin/Makefile.am (edit): Substitute @address@hidden
   * bin/autom4te.in: Call XFile::lock only if flock is
   implemented.

diff --git a/bin/Makefile.am b/bin/Makefile.am
index da65b85..c84f5ae 100644
--- a/bin/Makefile.am
+++ b/bin/Makefile.am
@@ -38,6 +38,7 @@ RELEASE_YEAR = \
 edit = sed \
        -e 's|@address@hidden|$(SHELL)|g' \
        -e 's|@address@hidden|$(PERL)|g' \
+       -e 's|@address@hidden|$(PERL_FLOCK)|g' \
        -e 's|@address@hidden|$(bindir)|g' \
        -e 's|@address@hidden|$(pkgdatadir)|g' \
        -e 's|@address@hidden|$(prefix)|g' \
diff --git a/bin/autom4te.in b/bin/autom4te.in
index 9f8faa3..6202a47 100644
--- a/bin/autom4te.in
+++ b/bin/autom4te.in
@@ -66,6 +66,8 @@ my $tcache;
 my $ocache;
 my $icache_file;
 
+my $flock_implemented = '@PERL_FLOCK@';
+
 # The macros to trace mapped to their format, as specified by the
 # user.
 my %trace;
@@ -978,7 +980,8 @@ if (! -d "$cache")
 # files, but the index is the first and last file to be updated, so
 # locking it is sufficient.
 $icache_file = new Autom4te::XFile $icache, O_RDWR|O_CREAT;
-$icache_file->lock (LOCK_EX);
+$icache_file->lock (LOCK_EX)
+  if ($flock_implemented eq "yes");
 
 # Read the cache index if available and older than autom4te itself.
 # If autom4te is younger, then some structures such as C4che might
diff --git a/configure.ac b/configure.ac
index 0d579f8..1241264 100644
--- a/configure.ac
+++ b/configure.ac
@@ -137,6 +137,22 @@ $PERL -e 'require 5.005_03;' || {
    AC_MSG_ERROR([Perl 5.005_03 or better is required])
 }
 
+# Find out whether the system supports flock
+# Note this test does not try to find out whether it works with this
+# particular file system.  It merely avoids us running flock on systems
+# where that bails out.
+AC_CACHE_CHECK([whether $PERL Fcntl::flock is implemented],
+  [ac_cv_perl_flock_implemented],
+  [echo lock me > conftest.fil
+   if $PERL -e 'use Fcntl ":flock"; flock("conftest.fil", LOCK_EX); 1;'; then
+     ac_cv_perl_flock_implemented=yes
+   else
+     ac_cv_perl_flock_implemented=no
+   fi
+   rm -f conftest.fil
+  ])
+AC_SUBST([PERL_FLOCK], [$ac_cv_perl_flock_implemented])
+
 
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