[config] Add Xcode 4 header search paths that take into account the n…

…ew Build Location value.

Before this change, Three20 compilation would fail whenever private
headers were imported because the headers were being copied to a new
location.

src/common/Configurations/Paths.xcconfig

@@ -23,5 +23,19 @@ SYMROOT = $(OBJROOT)/Products
 // Search Paths
 
 LIBRARY_SEARCH_PATHS    = $(STDLIB_LIBRARY)
-HEADER_SEARCH_PATHS     = $(STDLIB_HEADERS) "$(CONFIGURATION_BUILD_DIR)/../three20"
 
+// Header search paths: why we have three
+//
+// CONFIGURATION_BUILD_DIR is for when the headers are copied to Three20's Build folder.
+// Applies to:
+//  - Xcode 3.2.#
+//  - Xcode 4 with the Build Location preference set to "Place build products in locations
+//    specified by targets"
+//
+// BUILT_PRODUCTS_DIR is for Xcode 4 support only. We need two copies of a BUILT_PRODUCTS_DIR path
+//                    because on Archive builds, there is one extra folder placed between the
+//                    product and the public header path.
+// Applies to:
+//  - Xcode 4 only
+
+HEADER_SEARCH_PATHS     = $(STDLIB_HEADERS) "$(BUILT_PRODUCTS_DIR)/../three20" "$(BUILT_PRODUCTS_DIR)/../../three20" "$(CONFIGURATION_BUILD_DIR)/../../three20"