Configure: Add read_eval_file, a general purpose perl file reader/evaluator

It will return the last expression from the input file.

We also use this in read_config, which slightly changes what's
expected of Configurations/*.conf.  They do not have to assign
%targets specifically.  On the other hand, the table of configs MUST
be the last expression in each of those files.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4840)

Configurations/00-base-templates.conf

@@ -1,5 +1,5 @@
 # -*- Mode: perl -*-
-%targets=(
+my %targets=(
     DEFAULTS => {
 	template	=> 1,
 

Configurations/10-main.conf

@@ -144,7 +144,7 @@ sub vms_info {
     return $vms_info;
 }
 
-%targets = (
+my %targets = (
 
 #### Basic configs that should work on any 32-bit box
     "gcc" => {

Configurations/50-djgpp.conf

@@ -2,7 +2,7 @@
 # and rely entirely on the OpenSSL community to help is fine
 # tune and test.
 
-%targets = (
+my %targets = (
     "DJGPP" => {
         inherit_from     => [ asm("x86_asm") ],
         cc               => "gcc",

Configurations/50-haiku.conf

@@ -1,4 +1,4 @@
-%targets = (
+my %targets = (
     "haiku-common" => {
         template         => 1,
         cc               => "cc",

Configurations/50-masm.conf

@@ -7,7 +7,7 @@
 # proven to be daunting task. This is experimental target, for
 # production builds stick with [up-to-date version of] nasm.
 
-%targets = (
+my %targets = (
     "VC-WIN64A-masm" => {
         inherit_from    => [ "VC-WIN64-common", asm("x86_64_asm"),
                              sub { $disabled{shared} ? () : "x86_64_uplink" } ],

Configurations/90-team.conf

@@ -1,7 +1,7 @@
 ## -*- mode: perl; -*-
 ## Build configuration targets for openssl-team members
 
-%targets = (
+my %targets = (
     "purify" => {
         cc               => "purify gcc",
         cflags           => "-g -Wall",

Configure

@@ -2292,25 +2292,38 @@ sub add {
     sub { _add($separator, @_, @x) };
 }
 
+sub read_eval_file {
+    my $fname = shift;
+    my $content;
+    my @result;
+
+    open F, "< $fname" or die "Can't open '$fname': $!\n";
+    {
+        undef local $/;
+        $content = <F>;
+    }
+    close F;
+    {
+        local $@;
+
+        @result = ( eval $content );
+        warn $@ if $@;
+    }
+    return wantarray ? @result : $result[0];
+}
+
 # configuration reader, evaluates the input file as a perl script and expects
 # it to fill %targets with target configurations.  Those are then added to
 # %table.
 sub read_config {
     my $fname = shift;
-    open(CONFFILE, "< $fname")
-	or die "Can't open configuration file '$fname'!\n";
-    my $x = $/;
-    undef $/;
-    my $content = <CONFFILE>;
-    $/ = $x;
-    close(CONFFILE);
-    my %targets = ();
+    my %targets;
+
     {
 	# Protect certain tables from tampering
-	local %table = %::table;
+	local %table = ();
 
-	eval $content;
-	warn $@ if $@;
+	%targets = read_eval_file($fname);
     }
 
     # For each target, check that it's configured with a hash table.