https://github.com/jonathanstowe/TermReadKey/commit/1e3455135ac17f25ce0c4f01722a184a1fb5c4d1

From 1e3455135ac17f25ce0c4f01722a184a1fb5c4d1 Mon Sep 17 00:00:00 2001
From: Florian Weimer <fweimer@redhat.com>
Date: Fri, 25 Nov 2022 17:28:56 +0100
Subject: [PATCH] Configure.pm: Specify the return type of main as int

Implicit int is a language feature that was removed in the 1999
version of the C language.

I could not fix DeducePrototype because I do not see how it
can work reliably with this approach.  I don't think TermReadKey
uses this function.
--- a/Configure.pm
+++ b/Configure.pm
@@ -503,7 +503,7 @@ by the compiler is returned.
 =cut
 
 sub CheckHeader { #Find a header (or set of headers) that exists
-	ApplyHeaders("main(){}",@_);
+	ApplyHeaders("int main(){}",@_);
 }
 
 =head2 CheckStructure
@@ -516,7 +516,7 @@ properly will be returned. B<undef> will be returned if nothing succeeds.
 
 sub CheckStructure { # Check existance of a structure.
 	my($structname,@headers) = @_;
-	ApplyHeaders("main(){ struct $structname s;}",@headers);
+	ApplyHeaders("int main(){ struct $structname s;}",@headers);
 }
 
 =head2 CheckField
@@ -530,7 +530,7 @@ be returned if nothing succeeds.
 
 sub CheckField { # Check for the existance of specified field in structure
 	my($structname,$fieldname,@headers) = @_;
-	ApplyHeaders("main(){ struct $structname s1; struct $structname s2;
+	ApplyHeaders("int main(){ struct $structname s1; struct $structname s2;
 		     s1.$fieldname = s2.$fieldname; }",@headers);
 }
 
@@ -544,7 +544,7 @@ that symbol will be returned. B<undef> will be returned if nothing succeeds.
 
 sub CheckLSymbol { # Check for linkable symbol
 	my($symbol,@libs) = @_;
-	ApplyLibs("main() { void * f = (void *)($symbol); }",@libs);
+	ApplyLibs("int main() { void * f = (void *)($symbol); }",@libs);
 }
 
 =head2 CheckSymbol
@@ -558,7 +558,8 @@ B<undef> will be returned if nothing succeeds.
 
 sub CheckSymbol { # Check for linkable/header symbol
 	my($symbol,@lookup) = @_;
-	ApplyHeadersAndLibs("main() { void * f = (void *)($symbol); }",@lookup);
+	ApplyHeadersAndLibs("int main() { void * f = (void *)($symbol); }",
+			    @lookup);
 }
 
 =head2 CheckHSymbol
@@ -571,7 +572,7 @@ that symbol will be returned. B<undef> will be returned if nothing succeeds.
 
 sub CheckHSymbol { # Check for header symbol
 	my($symbol,@headers) = @_;
-	ApplyHeaders("main() { void * f = (void *)($symbol); }",@headers);
+	ApplyHeaders("int main() { void * f = (void *)($symbol); }",@headers);
 }
 
 =head2 CheckHPrototype (unexported)
@@ -591,7 +592,7 @@ sub CheckHPrototype { # Check for header prototype.
    # names function doesn't exist, this call will _succeed_. Caveat Utilitor.
 	my($function,$proto,@headers) = @_;
 	my(@proto) = @{$proto};
-	ApplyHeaders("main() { extern ".$proto[0]." $function(".
+	ApplyHeaders("int main() { extern ".$proto[0]." $function(".
                      join(",",@proto[1..$#proto])."); }",@headers);
 }
 
@@ -612,7 +613,8 @@ Example:
 sub GetSymbol { # Check for linkable/header symbol
 	my($symbol,$printf,$cast,@lookup) = @_,"","";
 	scalar(ApplyHeadersAndLibsAndExecute(
-		"main(){ printf(\"\%$printf\",($cast)($symbol));exit(0);}",@lookup));
+		"int main(){ printf(\"\%$printf\",($cast)($symbol));exit(0);}",
+		 @lookup));
 }
 
 =head2 GetTextSymbol
@@ -768,7 +770,7 @@ main(){ }");
 			"double","long double",
 			"char","unsigned char","short int","unsigned short int");
 
-		if(Compile("main(){flurfie a;}")) { @types = (); }
+		if(Compile("int main(){flurfie a;}")) { @types = (); }
 
 		$Verbose=0;
 
@@ -781,7 +783,7 @@ main(){ }");
 					if(Compile("
 extern void func($types[$i]);
 extern void func($types[$j]);
-main(){}")) {
+int main(){}")) {
                                             print "Removing type $types[$j] because it equals $types[$i]\n";
                                             splice(@types,$j,1);
                                             $j--;
@@ -795,7 +797,7 @@ main(){}")) {
 					if(Compile("
 $types[$i] func(void);
 extern $types[$j] func(void);
-main(){}")) {
+int main(){}")) {
 						print "Removing type $types[$j] because it equals $types[$i]\n";
 						splice(@types,$j,1);
 						$j--;
@@ -833,7 +835,7 @@ main(){}")) {
 	# Can we check the return type without worry about arguements?
 	if($checkreturn and (!$checknilargs or !$checkniletcargs)) {
 		for (@types) {
-			if(ApplyHeaders("extern $_ $function(". ($checknilargs?"...":"").");main(){}",[@headers])) {
+			if(ApplyHeaders("extern $_ $function(". ($checknilargs?"...":"").");int main(){}",[@headers])) {
 				$rettype = $_; # Great, we found the return type.
 				last;
 			}
@@ -847,9 +849,9 @@ main(){}")) {
 	my $numargs=-1;
 	my $varargs=0;
 	for (0..32) {
-			if(ApplyHeaders("main(){ $function(".join(",",("0") x $_).");}",@headers)) {
+			if(ApplyHeaders("int main(){ $function(".join(",",("0") x $_).");}",@headers)) {
 				$numargs=$_;
-				if(ApplyHeaders("main(){ $function(".join(",",("0") x ($_+1)).");}",@headers)) {
+				if(ApplyHeaders("int main(){ $function(".join(",",("0") x ($_+1)).");}",@headers)) {
 					$varargs=1;
 				}
 				last
@@ -865,7 +867,7 @@ main(){}")) {
 
 	if(@args>0 and !defined($rettype)) {
 		for (@types) {
-			if(defined(ApplyHeaders("extern $_ $function(".join(",",@args).");main(){}",[@headers]))) {
+			if(defined(ApplyHeaders("extern $_ $function(".join(",",@args).");int main(){}",[@headers]))) {
 				$rettype = $_; # Great, we found the return type.
 				last;
 			}