Update for Xcode 9.3.

xcode/libzip.xcodeproj/project.pbxproj

diff --git a/xcode/libzip.xcodeproj/project.pbxproj b/xcode/libzip.xcodeproj/project.pbxproj
index 1297f94..26ef3e4 100644
--- a/xcode/libzip.xcodeproj/project.pbxproj
+++ b/xcode/libzip.xcodeproj/project.pbxproj
@@ -1902,7 +1902,7 @@
 		4BDC71BF15B181DA00236D3C /* Project object */ = {
 			isa = PBXProject;
 			attributes = {
-				LastUpgradeCheck = 0920;
+				LastUpgradeCheck = 0930;
 				ORGANIZATIONNAME = NiH;
 				TargetAttributes = {
 					3D7E35361B3305FB00022624 = {
@@ -2852,11 +2852,13 @@
 				CLANG_WARN_BOOL_CONVERSION = YES;
 				CLANG_WARN_COMMA = YES;
 				CLANG_WARN_CONSTANT_CONVERSION = YES;
+				CLANG_WARN_DEPRECATED_OBJC_IMPLEMENTATIONS = YES;
 				CLANG_WARN_EMPTY_BODY = YES;
 				CLANG_WARN_ENUM_CONVERSION = YES;
 				CLANG_WARN_IMPLICIT_SIGN_CONVERSION = YES;
 				CLANG_WARN_INFINITE_RECURSION = YES;
 				CLANG_WARN_INT_CONVERSION = YES;
+				CLANG_WARN_OBJC_IMPLICIT_RETAIN_SELF = YES;
 				CLANG_WARN_RANGE_LOOP_ANALYSIS = YES;
 				CLANG_WARN_STRICT_PROTOTYPES = YES;
 				CLANG_WARN_SUSPICIOUS_IMPLICIT_CONVERSION = YES;
@@ -2900,11 +2902,13 @@
 				CLANG_WARN_BOOL_CONVERSION = YES;
 				CLANG_WARN_COMMA = YES;
 				CLANG_WARN_CONSTANT_CONVERSION = YES;
+				CLANG_WARN_DEPRECATED_OBJC_IMPLEMENTATIONS = YES;
 				CLANG_WARN_EMPTY_BODY = YES;
 				CLANG_WARN_ENUM_CONVERSION = YES;
 				CLANG_WARN_IMPLICIT_SIGN_CONVERSION = YES;
 				CLANG_WARN_INFINITE_RECURSION = YES;
 				CLANG_WARN_INT_CONVERSION = YES;
+				CLANG_WARN_OBJC_IMPLICIT_RETAIN_SELF = YES;
 				CLANG_WARN_RANGE_LOOP_ANALYSIS = YES;
 				CLANG_WARN_STRICT_PROTOTYPES = YES;
 				CLANG_WARN_SUSPICIOUS_IMPLICIT_CONVERSION = YES;

xcode/libzip.xcodeproj/project.xcworkspace/xcshareddata/IDEWorkspaceChecks.plist

diff --git a/xcode/libzip.xcodeproj/project.xcworkspace/xcshareddata/IDEWorkspaceChecks.plist b/xcode/libzip.xcodeproj/project.xcworkspace/xcshareddata/IDEWorkspaceChecks.plist
new file mode 100644
index 0000000..18d9810
--- /dev/null
+++ b/xcode/libzip.xcodeproj/project.xcworkspace/xcshareddata/IDEWorkspaceChecks.plist
@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>IDEDidComputeMac32BitWarning</key>
+	<true/>
+</dict>
+</plist>