Add in.substitution={strict|lax}

In the strict mode every substitution symbol is expected to start a
substitution with the double symbol (e.g., $$) serving as an
escape sequence.

In the lax mode a pair of substitution symbols is only treated as a
substitution if what's between them looks like a build2 variable
name (i.e., doesn't contain spaces, etc). Everything else,
including unterminated substitution symbols is copied as is. Note
also that in this mode the double symbol is not treated as an
escape sequence.

The lax mode is mostly useful when trying to reuse existing .in
files, for example from autoconf. Note, however, that the lax mode
is still stricter than the autoconf's semantics which also leaves
unknown substitutions as is.

1 files changed, 29 insertions, 7 deletions

diff --git a/build2/version/init.cxx b/build2/version/init.cxx
index c0ce4cd..69d7401 100644
--- a/build2/version/init.cxx
+++ b/build2/version/init.cxx
@@ -272,15 +272,37 @@ namespace build2
         }
       }
 
-      // Enter variables. Note: some overridable, some not.
+      // Enter variables.
       //
-      auto& vp (var_pool.rw (rs));
+      {
+        auto& vp (var_pool.rw (rs));
 
-      // Alternative variable substitution symbols.
-      //
-      // @@ Note: should be moved to the 'in' module once we have it.
-      //
-      m.in_symbol = &vp.insert<string> ("in.symbol");
+        // @@ Note: these should be moved to the 'in' module once we have it.
+        //
+
+        // Alternative variable substitution symbol.
+        //
+        m.in_symbol = &vp.insert<string> ("in.symbol");
+
+        // Substitution mode. Valid values are 'strict' (default) and 'lax'.
+        // In the strict mode every substitution symbol is expected to start a
+        // substitution with the double symbol (e.g., $$) serving as an
+        // escape sequence.
+        //
+        // In the lax mode a pair of substitution symbols is only treated as a
+        // substitution if what's between them looks like a build2 variable
+        // name (i.e., doesn't contain spaces, etc). Everything else,
+        // including unterminated substitution symbols is copied as is. Note
+        // also that in this mode the double symbol is not treated as an
+        // escape sequence.
+        //
+        // The lax mode is mostly useful when trying to reuse existing .in
+        // files, for example from autoconf. Note, however, that the lax mode
+        // is still stricter than the autoconf's semantics which also leaves
+        // unknown substitutions as is.
+        //
+        m.in_substitution = &vp.insert<string> ("in.substitution");
+      }
 
       // Register rules.
       //