Let Xcode Project build static library - objective-c

Question

I have a Core Library for my ios projects. I use this library in several projects.
When building the library, I copied the .a -file and the header-files into several xcode projects. Everything worked fine.
Now I would like that each project, which uses the library, build the library on their own.
I tried to use this runscript:
# Build Core library first
CORE="${PWD}/../Core"
IDENT="PROJECTIDENT"
PROJ=$CORE/Core.xcodeproj
CORE_LIB="${PWD}/Classes/Core"
mkdir -p "${CORE_LIB}"
#rm -rf "${CORE_LIB}/"*
mkdir -p "/tmp/${IDENT}"
IS_EMPTY=`ls "${CORE_LIB}" | wc -l`
if [ "true" == ${ALREADYINVOKED:-false} ]; then
echo "ALREADY INVOKED"
else
# CRITICAL:
# Prevent infinite recursion (Xcode sucks)
export ALREADYINVOKED="true"
SDK_VERSION=$(echo ${SDK_NAME} | grep -o '.\{3\}$')
OUT_DIR="/tmp/${IDENT}"
EXEC_NAME="libCore.a"
UNIVERSAL_DIR="${OUT_DIR}/Universal"
rm -rf "${UNIVERSAL_DIR}"
mkdir -p "${UNIVERSAL_DIR}"
CONFIG="Debug"
if [ ${CONFIGURATION} != "Debug" ]; then
CONFIG="Release"
fi
echo "${CONFIG}"
# Create build
DID_COMPILE=`xcodebuild -project $PROJ -target Core -configuration "${CONFIG}" -sdk "${PLATFORM_NAME}${SDK_VERSION}" build RUN_CLANG_STATIC_ANALYZER=NO BUILD_DIR="${OUT_DIR}" BUILD_ROOT="${OUT_DIR}" EXECUTABLE_NAME="${EXEC_NAME}" | grep "CompileC"`
if [ ! -z "${DID_COMPILE}" ] || [ ${IS_EMPTY} -eq 0 ]; then
# Copy Headers
cp "${OUT_DIR}/${CONFIG}-${PLATFORM_NAME}/usr/local/include/"* "${UNIVERSAL_DIR}"
cp "${OUT_DIR}/${CONFIG}-${PLATFORM_NAME}/${EXEC_NAME}"* "${UNIVERSAL_DIR}"
# Finally we have a fat static library with .a and .h files
# now we copy them to the Project
rm -rf "${CORE_LIB}"
cp -Rf "${UNIVERSAL_DIR}" "${CORE_LIB}"
fi
fi
This script works but only every second time.
E.g. I make a change in a Library Class, and built, the main project is not compiled while the Library dependency get refreshed.
Then I make another change to the Library, build again, and all changes are usable in the main probject.
But always only each second build....
How can I fix this?