Define IOSSDK in case it has not been defined yet

Makefile.libretro

@@ -29,9 +29,13 @@ else ifeq ($(platform), osx)
    SHARED := -dynamiclib
 else ifeq ($(platform), ios)
    TARGET := $(TARGET_NAME)_libretro_ios.dylib
-	fpic := -fPIC
-	SHARED := -dynamiclib
-	DEFINES := -DIOS
+   fpic := -fPIC
+   SHARED := -dynamiclib
+   DEFINES := -DIOS
+
+ifeq ($(IOSSDK),)
+   IOSSDK := $(shell xcrun -sdk iphoneos -show-sdk-path)
+endif
 	CC = clang -arch armv7 -isysroot $(IOSSDK)
 else ifeq ($(platform), qnx)
 	TARGET := $(TARGET_NAME)_libretro_qnx.so