portability for systems without /usr/local on package nsis

src/nsis.mk

@@ -20,8 +20,8 @@ define $(PKG)_BUILD
     $(SED) 's,this->SetIcon(wxICON(nsisicon));,,' -i '$(1)/Contrib/NSIS Menu/nsismenu/nsismenu.cpp'
     cd '$(1)' && scons \
         PREFIX='$(PREFIX)' \
-        APPEND_CPPPATH='/usr/local/include' \
-        APPEND_LIBPATH='/usr/local/lib' \
+        `[ -d /usr/local/include ] && echo APPEND_CPPPATH=/usr/local/include` \
+        `[ -d /usr/local/lib ]     && echo APPEND_LIBPATH=/usr/local/lib` \
         SKIPUTILS='NSIS Menu' \
         install
 endef