add include in cmake lists

[Lixue9jiu]

This allows the end user to do

add_subdirectory(enet)

target_link_libraries(Project
...
        enet
)

and get the include directories set up by CMake, without defining the include directory manually.
It provides better integration with the CMake build system.

[bjorn]

Thanks, this is a welcome modernization! One suggestion: wrap the path in a BUILD_INTERFACE generator expression. As written it causes no error today because the install(TARGETS enet ...) rule has no EXPORT, but as soon as an export set is added, CMake will fail with "INTERFACE_INCLUDE_DIRECTORIES property contains a path prefixed in the source directory". Adding the generator expressions now costs nothing and keeps that door open (they are supported at the declared CMake 2.8.12 minimum).

Suggested change

	target_include_directories(enet
	PUBLIC
	${PROJECT_SOURCE_DIR}/include
	)
	target_include_directories(enet PUBLIC
	$<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>
	$<INSTALL_INTERFACE:include>
	)

This also matches the indentation style used elsewhere in the file, such as the install(TARGETS enet ...) block.