Xcode – symbol(s) not found for architecture x86_64 (iOS Lib)

I had the same trouble with building static library.
Finally I have found the basic solution. (You need to build universal library for x86_64/armv7/armv7s/arm64)

1) Click on the project file
2) Click on the target
3) Open "Build Phases"
4) Open "Run Script"
5) Add "/bin/sh" and the script below

##########################################
#
# c.f. http://stackoverflow.com/questions/3520977/build-fat-static-library-device-simulator-using-xcode-and-sdk-4
#
# Version 2.7
#
# Latest Change:
# - Supports iPhone 5 / iPod Touch 5 (uses Apple's workaround to lipo bug)
#
# Purpose:
#   Automatically create a Universal static library for iPhone + iPad + iPhone Simulator from within XCode
#
# Author: Adam Martin - http://twitter.com/redglassesapps
# Based on: original script from Eonil (main changes: Eonil's script WILL NOT WORK in Xcode GUI - it WILL CRASH YOUR COMPUTER)
#

set -e
set -o pipefail

#################[ Tests: helps workaround any future bugs in Xcode ]########
#
DEBUG_THIS_SCRIPT="false"

if [ $DEBUG_THIS_SCRIPT = "true" ]
then
echo "########### TESTS #############"
echo "Use the following variables when debugging this script; note that they may change on recursions"
echo "BUILD_DIR = $BUILD_DIR"
echo "BUILD_ROOT = $BUILD_ROOT"
echo "CONFIGURATION_BUILD_DIR = $CONFIGURATION_BUILD_DIR"
echo "BUILT_PRODUCTS_DIR = $BUILT_PRODUCTS_DIR"
echo "CONFIGURATION_TEMP_DIR = $CONFIGURATION_TEMP_DIR"
echo "TARGET_BUILD_DIR = $TARGET_BUILD_DIR"
fi

#####################[ part 1 ]##################
# First, work out the BASESDK version number (NB: Apple ought to report this, but they hide it)
#    (incidental: searching for substrings in sh is a nightmare! Sob)

SDK_VERSION=$(echo ${SDK_NAME} | grep -o '.\{3\}$')

# Next, work out if we're in SIM or DEVICE

if [ ${PLATFORM_NAME} = "iphonesimulator" ]
then
OTHER_SDK_TO_BUILD=iphoneos${SDK_VERSION}
else
OTHER_SDK_TO_BUILD=iphonesimulator${SDK_VERSION}
fi

echo "XCode has selected SDK: ${PLATFORM_NAME} with version: ${SDK_VERSION} (although back-targetting: ${IPHONEOS_DEPLOYMENT_TARGET})"
echo "...therefore, OTHER_SDK_TO_BUILD = ${OTHER_SDK_TO_BUILD}"
#
#####################[ end of part 1 ]##################

#####################[ part 2 ]##################
#
# IF this is the original invocation, invoke WHATEVER other builds are required
#
# Xcode is already building ONE target...
#
# ...but this is a LIBRARY, so Apple is wrong to set it to build just one.
# ...we need to build ALL targets
# ...we MUST NOT re-build the target that is ALREADY being built: Xcode WILL CRASH YOUR COMPUTER if you try this (infinite recursion!)
#
#
# So: build ONLY the missing platforms/configurations.

if [ "true" == ${ALREADYINVOKED:-false} ]
then
echo "RECURSION: I am NOT the root invocation, so I'm NOT going to recurse"
else
# CRITICAL:
# Prevent infinite recursion (Xcode sucks)
export ALREADYINVOKED="true"

echo "RECURSION: I am the root ... recursing all missing build targets NOW..."
echo "RECURSION: ...about to invoke: xcodebuild -configuration \"${CONFIGURATION}\" -project \"${PROJECT_NAME}.xcodeproj\" -target \"${TARGET_NAME}\" -sdk \"${OTHER_SDK_TO_BUILD}\" ${ACTION} RUN_CLANG_STATIC_ANALYZER=NO" BUILD_DIR=\"${BUILD_DIR}\" BUILD_ROOT=\"${BUILD_ROOT}\" SYMROOT=\"${SYMROOT}\"

xcodebuild -configuration "${CONFIGURATION}" -project "${PROJECT_NAME}.xcodeproj" -target "${TARGET_NAME}" -sdk "${OTHER_SDK_TO_BUILD}" ${ACTION} RUN_CLANG_STATIC_ANALYZER=NO BUILD_DIR="${BUILD_DIR}" BUILD_ROOT="${BUILD_ROOT}" SYMROOT="${SYMROOT}"

ACTION="build"

#Merge all platform binaries as a fat binary for each configurations.

# Calculate where the (multiple) built files are coming from:
CURRENTCONFIG_DEVICE_DIR=${SYMROOT}/${CONFIGURATION}-iphoneos
CURRENTCONFIG_SIMULATOR_DIR=${SYMROOT}/${CONFIGURATION}-iphonesimulator

echo "Taking device build from: ${CURRENTCONFIG_DEVICE_DIR}"
echo "Taking simulator build from: ${CURRENTCONFIG_SIMULATOR_DIR}"

CREATING_UNIVERSAL_DIR=${SYMROOT}/${CONFIGURATION}-universal
echo "...I will output a universal build to: ${CREATING_UNIVERSAL_DIR}"

# ... remove the products of previous runs of this script
#      NB: this directory is ONLY created by this script - it should be safe to delete!

rm -rf "${CREATING_UNIVERSAL_DIR}"
mkdir "${CREATING_UNIVERSAL_DIR}"

#
echo "lipo: for current configuration (${CONFIGURATION}) creating output file: ${CREATING_UNIVERSAL_DIR}/${EXECUTABLE_NAME}"
xcrun -sdk iphoneos lipo -create -output "${CREATING_UNIVERSAL_DIR}/${EXECUTABLE_NAME}" "${CURRENTCONFIG_DEVICE_DIR}/${EXECUTABLE_NAME}" "${CURRENTCONFIG_SIMULATOR_DIR}/${EXECUTABLE_NAME}"

#########
#
# Added: StackOverflow suggestion to also copy "include" files
#    (untested, but should work OK)
#
echo "Fetching headers from ${PUBLIC_HEADERS_FOLDER_PATH}"
echo "  (if you embed your library project in another project, you will need to add"
echo "   a "User Search Headers" build setting of: (NB INCLUDE THE DOUBLE QUOTES BELOW!)"
echo '        "$(TARGET_BUILD_DIR)/usr/local/include/"'
if [ -d "${CURRENTCONFIG_DEVICE_DIR}${PUBLIC_HEADERS_FOLDER_PATH}" ]
then
mkdir -p "${CREATING_UNIVERSAL_DIR}${PUBLIC_HEADERS_FOLDER_PATH}"
# * needs to be outside the double quotes?
cp -r "${CURRENTCONFIG_DEVICE_DIR}${PUBLIC_HEADERS_FOLDER_PATH}"* "${CREATING_UNIVERSAL_DIR}${PUBLIC_HEADERS_FOLDER_PATH}"
fi
fi

6) Hit "cmd + B" (Build Project)

7) Open Product in Finder

8) Navigate 1 directory up (“cmd + ↑”), and you will see "Release-universal" directory.

There will be your "fat/universal" library, You are ready to go!