I'm using the makefile below for my library and it works flawless even with XCode 5 and the iOS7 SDK.
XBUILD=/Applications/Xcode.app/Contents/Developer/usr/bin/xcodebuild
PROJECT_ROOT=.
PROJECT=$(PROJECT_ROOT)/GIFLibFrontEnd.xcodeproj
TARGET=GIFLibFrontEnd
all: libUniversal.a
libi386.a:
$(XBUILD) -project $(PROJECT) -target $(TARGET) -sdk iphonesimulator -configuration Release clean build
-mv $(PROJECT_ROOT)/build/Release-iphonesimulator/lib$(TARGET).a $@
libArmv7.a:
$(XBUILD) -project $(PROJECT) -target $(TARGET) -sdk iphoneos -arch armv7 -configuration Release clean build
-mv $(PROJECT_ROOT)/build/Release-iphoneos/lib$(TARGET).a $@
libArmv7s.a:
$(XBUILD) -project $(PROJECT) -target $(TARGET) -sdk iphoneos -arch armv7s -configuration Release clean build
-mv $(PROJECT_ROOT)/build/Release-iphoneos/lib$(TARGET).a $@
libArm64.a:
$(XBUILD) -project $(PROJECT) -target $(TARGET) -sdk iphoneos -arch arm64 -configuration Release clean build
-mv $(PROJECT_ROOT)/build/Release-iphoneos/lib$(TARGET).a $@
libUniversal.a: libi386.a libArmv7.a libArmv7s.a libArm64.a
lipo -create -output lib$(TARGET)Universal.a $^
clean:
-rm -f *.a *.dll
-rm -rf build