Add option to not use bundled TinyXML-2, PhysFS, and UTF8-CPP

If you configure the build with -DBUNDLE_DEPENDENCIES=OFF, then VVVVVV
will dynamically link with TinyXML-2 and PhysicsFS instead of using the
bundled source code in third_party/ and statically linking with them.

Unfortunately, it doesn't seem like distros package LodePNG, and LodePNG
isn't intended to be packaged, so we can't dynamically link with it, nor
can we use some system LodePNG header files somewhere else because those
don't exist.

UTF8-CPP is a special case, because no matter what, it's going to be
statically linked with the binary (it doesn't come as a shared object
file in any way). So with -DBUNDLE_DEPENDENCIES=OFF, we will use the
system UTF8-CPP header files instead of the bundled ones, but it will
still be statically linked in with the binary.

The main motivation for doing this is so if VVVVVV ever gets packaged in
distros, distro maintainers would be more likely to accept it if there
was an option to compile the game without bundled dependencies. Also, it
discourages modifying the third-party dependencies we have, because it's
always possible for someone to compile those dependencies without our
changes, with this CMake option.

desktop_version/CMakeLists.txt

@@ -7,6 +7,8 @@ CMAKE_MINIMUM_REQUIRED(VERSION 2.8.12)
 OPTION(ENABLE_WARNINGS "Enable compilation warnings" ON)
 OPTION(ENABLE_WERROR "Treat compilation warnings as errors" OFF)
 
+SET(BUNDLE_DEPENDENCIES ON CACHE BOOL "Use bundled TinyXML-2, PhysicsFS, and UTF8-CPP (if disabled, TinyXML-2 and PhysicsFS will be dynamically linked, LodePNG and UTF8-CPP will still be statically linked)")
+
 SET(CUSTOM_LEVEL_SUPPORT ENABLED CACHE STRING "Optionally disable playing and/or editing of custom levels")
 SET_PROPERTY(CACHE CUSTOM_LEVEL_SUPPORT PROPERTY STRINGS ENABLED NO_EDITOR DISABLED)
 
@@ -74,6 +76,7 @@ IF(NOT WIN32)
 ENDIF()
 
 # Include Directories
+IF(BUNDLE_DEPENDENCIES)
 	INCLUDE_DIRECTORIES(
 		src
 		../third_party/tinyxml2
@@ -81,6 +84,12 @@ INCLUDE_DIRECTORIES(
 		../third_party/lodepng
 		../third_party/utfcpp/source
 	)
+ELSE()
+	INCLUDE_DIRECTORIES(
+		src
+		../third_party/lodepng
+	)
+ENDIF()
 
 # Source Lists
 SET(VVV_SRC
@@ -233,9 +242,7 @@ ENDIF()
 SET_PROPERTY(TARGET VVVVVV PROPERTY CXX_STANDARD 98)
 SET_PROPERTY(TARGET VVVVVV PROPERTY CXX_EXTENSIONS FALSE)
 
-# Library information
+# Unfortunately, it doesn't seem like distros package LodePNG
-ADD_LIBRARY(tinyxml2-static STATIC ${XML2_SRC})
-ADD_LIBRARY(physfs-static STATIC ${PFS_SRC} ${PFSP_SRC})
 ADD_LIBRARY(lodepng-static STATIC ${PNG_SRC})
 
 TARGET_COMPILE_DEFINITIONS(lodepng-static PRIVATE
@@ -244,8 +251,16 @@ TARGET_COMPILE_DEFINITIONS(lodepng-static PRIVATE
 	-DLODEPNG_NO_COMPILE_ENCODER
 )
 
-# Static Dependencies
+IF(BUNDLE_DEPENDENCIES)
+	ADD_LIBRARY(tinyxml2-static STATIC ${XML2_SRC})
+	ADD_LIBRARY(physfs-static STATIC ${PFS_SRC} ${PFSP_SRC})
+
 	TARGET_LINK_LIBRARIES(VVVVVV physfs-static tinyxml2-static lodepng-static)
+ELSE()
+	FIND_PACKAGE(utf8cpp CONFIG)
+
+	TARGET_LINK_LIBRARIES(VVVVVV physfs tinyxml2 utf8cpp lodepng-static)
+ENDIF()
 
 # SDL2 Dependency (Detection pulled from FAudio)
 if (DEFINED SDL2_INCLUDE_DIRS AND DEFINED SDL2_LIBRARIES)