Enable -Wshadow by default

The code base is almost shadow-free now, so this won't add much noise to the compiles.
Problems I know about:
 lxt{,2}_write.c:  patch sent upstream
 cflexor.c:  exposes gray area of name space boundary

Makefile.in

@@ -81,7 +81,7 @@ PS2PDF = @PS2PDF@
 GIT = @GIT@
 
 CPPFLAGS = @DEFS@ -I. -I$(srcdir) -DVERSION='"$(VERSION)"' @CPPFLAGS@
-CXXFLAGS = -Wall @CXXFLAGS@
+CXXFLAGS = -Wall -Wshadow @CXXFLAGS@
 PICFLAGS = @PICFLAG@
 LDFLAGS = @rdynamic@ @LDFLAGS@