build.info: Implement simply substitutions in variable values

Use case: having a variable with multiple source files in its value,
and wanting to refer to the corresponding object file.

    $SRCS=foo.c bar.c
    SOURCE[program]=$SRCS
    DEPEND[${SRCS/.c/.o}]=prog.h

    GENERATE[prog.h]=...

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/11185)

2 files changed, 60 insertions, 5 deletions

diff --git a/Configure b/Configure
index 737a471c1d..19c16f134c 100755
--- a/Configure
+++ b/Configure
@@ -1807,23 +1807,62 @@ if ($builder eq "unified") {
         # contains a dollar sign, it had better be escaped, or it will be
         # taken for a variable name prefix.
         my %variables = ();
-        my $variable_re = qr/\$(?P<VARIABLE>[[:alpha:]][[:alnum:]_]*)/;
+        # Variable name syntax
+        my $variable_name_re = qr/(?P<VARIABLE>[[:alpha:]][[:alnum:]_]*)/;
+        # Value modifier syntaxes
+        my $variable_subst_re = qr/\/(?P<RE>(?:\\\/|.)*?)\/(?P<SUBST>.*?)/;
+        # Put it all together
+        my $variable_re = qr/\$
+                             (?|
+                                 # Simple case, just the name
+                                 ${variable_name_re}
+                             |
+                                 # Expressive case, with braces and possible
+                                 # modifier expressions
+                                 \{
+                                 ${variable_name_re}
+                                 (?:
+                                     # Pile on modifier expressions,
+                                     # separated by |
+                                     ${variable_subst_re}
+                                 )
+                                 \}
+                             )/x;
         my $expand_variables = sub {
             my $value = '';
             my $value_rest = shift;
 
             if ($ENV{CONFIGURE_DEBUG_VARIABLE_EXPAND}) {
                 print STDERR
-                    "DEBUG[\$expand_variables] Parsed '$value_rest' into:\n"
+                    "DEBUG[\$expand_variables] Parsed '$value_rest' ...\n"
             }
             while ($value_rest =~ /(?<!\\)${variable_re}/) {
-                $value .= $`;
-                $value .= $variables{$+{VARIABLE}};
                 $value_rest = $';
+                $value .= $`;
+
+                my $variable_value = $variables{$+{VARIABLE}};
+
+                # Process modifier expressions, if present
+                if (defined $+{RE}) {
+                    # We must save important %+ values, because the s///
+                    # below clears them
+                    my $re = $+{RE};
+                    my $subst = $+{SUBST};
+
+                    $variable_value =~ s/\Q$re\E/$subst/g;
+
+                    if ($ENV{CONFIGURE_DEBUG_VARIABLE_EXPAND}) {
+                        print STDERR
+                            "DEBUG[\$expand_variables] ... and substituted ",
+                            "'$re' with '$subst'\n";
+                    }
+                }
+
+                $value .= $variable_value;
             }
             if ($ENV{CONFIGURE_DEBUG_VARIABLE_EXPAND}) {
                 print STDERR
-                    "DEBUG[\$expand_variables] ... '$value$value_rest'\n";
+                    "DEBUG[\$expand_variables] ... into: '$value$value_rest'\n";
             }
             return $value . $value_rest;
         };
diff --git a/doc/internal/man7/build.info.pod b/doc/internal/man7/build.info.pod
index c2020a432f..f6ca49067d 100644
--- a/doc/internal/man7/build.info.pod
+++ b/doc/internal/man7/build.info.pod
@@ -270,6 +270,22 @@ part of is tokenized>.
 
 I<Variable assignment values are not tokenized.>
 
+Variable references can be one of:
+
+=over 4
+
+=item B<$>I<NAME> or B<${>I<NAME>B<}>
+
+Simple reference; the variable reference is replaced with its value,
+verbatim.
+
+=item B<${>I<NAME>B</>I<str>B</>I<subst>B<}>
+
+Substitution reference; the variable reference is replaced with its
+value, modified by replacing all occurences of I<str> with I<subst>.
+
+=back
+
 =head2 Scope
 
 Most of the statement values are accumulated globally from all the