CMake: Use find_package and imported targets for Iconv

Fixes us forgetting to add its include directories, which could result in linking to a dylib from MacPorts while using the system's header, and failing to link because they use different function names

Externals/libiconv-1.14/CMakeLists.txt

@@ -1,10 +1,13 @@
-include_directories(include)
-include_directories(libcharset/include)
-
-set(SRCS	lib/iconv.c
-		lib/relocatable.c
-		libcharset/lib/localcharset.c
+add_library(iconv STATIC
+	lib/iconv.c
+	lib/relocatable.c
+	libcharset/lib/localcharset.c
+)
+target_include_directories(iconv
+	PUBLIC
+		include
+	PRIVATE
+		libcharset/include
 )
-
-add_library(iconv STATIC ${SRCS})
 dolphin_disable_warnings_msvc(iconv)
+add_library(Iconv::Iconv ALIAS iconv)