Eliminated spurious "wx-config not found" messages.

src/build/wxWidgets.gmake

 ifndef WX_CONFIG
- ifeq ($(shell wx-config --cflags > /dev/null 2>&1 && echo YES),YES)
+ ifeq ($(shell sh -c 'wx-config --cflags' > /dev/null 2>&1 && echo YES),YES)
   WX_CONFIG := wx-config
   WITH_WX	:=	1
  else
-  ifeq ($(shell wxgtk2u-2.8-config --cflags > /dev/null 2>&1 && echo YES),YES)
-   WX_CONFIG := wx13-config
+  ifeq ($(shell sh -c 'wxgtk2u-2.8-config --cflags' > /dev/null 2>&1 && echo YES),YES)
+   WX_CONFIG := wxgtk2u-2.8-config
    WITH_WX	:=	1
   endif
  endif
 endif
 
 ifdef WX_CONFIG
- ifeq ($(shell $(WX_CONFIG) --version=2.8 --query-version),2.8)
+ ifeq ($(shell $(WX_CONFIG) --version=2.8 --query-version 2>&1),2.8)
   WITH_WX	:=	1
  endif
 endif