Fix failure of ./configure --with-tcl-binding --with-xml-support

If pkg-config has found tcl.pc, then tclConfig.sh is not executed
and $TCL_INCLUDE_SPEC is empty so the test for tcl.h fails.
This patch moves the test inside the "else" case where the variable
is always defined.
The is a bug exposed by the fact that --with-xml-support redefines
the macro PKG_CHECK_MODULES.
Fix verified moving aside /usr/lib/x86_64-linux-gnu/pkgconfig/tcl.pc

Fixes:
configure: error: Unable to find Tcl headers

configure.ac

@@ -637,14 +637,14 @@ AS_IF([test x"${build_tcl}" = "xyes"],[
             AC_MSG_WARN([Unable to find Tcl pkgconfig])
             SC_PATH_TCLCONFIG
             SC_LOAD_TCLCONFIG
-        ])
 
-    tcl_save_CPPFLAGS=$CPPFLAGS
-    CPPFLAGS="$CPPFLAGS $TCL_INCLUDE_SPEC"
-    AC_CHECK_HEADERS([tcl.h],
-	[],
-	[AC_MSG_ERROR([Unable to find Tcl headers])])
-    CPPFLAGS=$tcl_save_CPPFLAGS
+            tcl_save_CPPFLAGS=$CPPFLAGS
+            CPPFLAGS="$CPPFLAGS $TCL_INCLUDE_SPEC"
+            AC_CHECK_HEADERS([tcl.h],
+            [],
+            [AC_MSG_ERROR([Unable to find Tcl headers])])
+            CPPFLAGS=$tcl_save_CPPFLAGS
+        ])
 
     BINDING_LIST="${BINDING_LIST} tcl"
     BINDING_ALL="${BINDING_ALL} all-tcl"