Add builtin constants ‘langVersion’ and ‘nixVersion’

The integer constant ‘langVersion’ denotes the current language
version.  It gets increased every time a language feature is
added/changed/removed.  It's currently 1.

The string constant ‘nixVersion’ contains the current Nix version,
e.g. "1.2pre2980_9de6bc5".

src/libexpr/primops.cc

@@ -1143,6 +1143,16 @@ void EvalState::createBaseEnv()
     mkString(v, settings.thisSystem.c_str());
     addConstant("__currentSystem", v);
 
+    mkString(v, nixVersion.c_str());
+    addConstant("__nixVersion", v);
+
+    /* Language version.  This should be increased every time a new
+       language feature gets added.  It's not necessary to increase it
+       when primops get added, because you can just use `builtins ?
+       primOp' to check. */
+    mkInt(v, 1);
+    addConstant("__langVersion", v);
+
     // Miscellaneous
     addPrimOp("import", 1, prim_import);
     addPrimOp("isNull", 1, prim_isNull);

src/libmain/Makefile.am

@@ -7,6 +7,5 @@ libmain_la_LIBADD = ../libstore/libstore.la @BDW_GC_LIBS@
 pkginclude_HEADERS = shared.hh
 
 AM_CXXFLAGS = \
- -DNIX_VERSION=\"$(VERSION)\" \
  -I$(srcdir)/.. -I$(srcdir)/../libutil \
  -I$(srcdir)/../libstore

src/libmain/shared.cc

@@ -180,7 +180,7 @@ static void initAndRun(int argc, char * * argv)
             return;
         }
         else if (arg == "--version") {
-            std::cout << format("%1% (Nix) %2%") % programId % NIX_VERSION << std::endl;
+            std::cout << format("%1% (Nix) %2%") % programId % nixVersion << std::endl;
             return;
         }
         else if (arg == "--keep-failed" || arg == "-K")

src/libstore/Makefile.am

@@ -23,6 +23,7 @@ AM_CXXFLAGS = -Wall \
  -DNIX_CONF_DIR=\"$(sysconfdir)/nix\" \
  -DNIX_LIBEXEC_DIR=\"$(libexecdir)\" \
  -DNIX_BIN_DIR=\"$(bindir)\" \
+ -DNIX_VERSION=\"$(VERSION)\" \
  -I$(srcdir)/.. -I$(srcdir)/../libutil \
  -I$(srcdir)/../libstore
 

src/libstore/globals.cc

@@ -197,4 +197,7 @@ Settings::SettingsMap Settings::getOverrides()
 }
 
 
+const string nixVersion = NIX_VERSION;
+
+
 }

src/libstore/globals.hh

@@ -190,4 +190,7 @@ private:
 extern Settings settings;
 
 
+extern const string nixVersion;
+
+
 }