Fixed compilation on Ubuntu 18.04LTS and 20.04LTS

include/prereq-build.mk

@@ -145,7 +145,7 @@ $(eval $(call SetupHostCommand,svn,Please install the Subversion client, \
 	svn --version | grep Subversion))
 
 $(eval $(call SetupHostCommand,git,Please install Git (git-core) >= 1.6.5, \
-	git clone 2>&1 | grep -- --recursive))
+	git --exec-path | xargs -I % -- grep -q -- --recursive %/git-submodule))
 
 $(eval $(call SetupHostCommand,file,Please install the 'file' package, \
 	file --version 2>&1 | grep file))

tools/automake/patches/010-automake-port-to-Perl-5.22-and-later.patch

@@ -0,0 +1,30 @@
+From 13f00eb4493c217269b76614759e452d8302955e Mon Sep 17 00:00:00 2001
+From: Paul Eggert <[email protected]>
+Date: Thu, 31 Mar 2016 16:35:29 -0700
+Subject: [PATCH] automake: port to Perl 5.22 and later
+
+Without this change, Perl 5.22 complains "Unescaped left brace in
+regex is deprecated" and this is planned to become a hard error in
+Perl 5.26.  See:
+http://search.cpan.org/dist/perl-5.22.0/pod/perldelta.pod#A_literal_%22{%22_should_now_be_escaped_in_a_pattern
+* bin/automake.in (substitute_ac_subst_variables): Escape left brace.
+---
+ bin/automake.in | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/bin/automake.in b/bin/automake.in
+index a3a0aa318..2c8f31e14 100644
+--- a/bin/automake.in
++++ b/bin/automake.in
+@@ -3878,7 +3878,7 @@ sub substitute_ac_subst_variables_worker
+ sub substitute_ac_subst_variables
+ {
+   my ($text) = @_;
+-  $text =~ s/\${([^ \t=:+{}]+)}/substitute_ac_subst_variables_worker ($1)/ge;
++  $text =~ s/\$[{]([^ \t=:+{}]+)}/substitute_ac_subst_variables_worker ($1)/ge;
+   return $text;
+ }
+
+-- 
+2.13.1
+