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/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;
+
+
 }