Add compatibility to vcpkg buildsystem

[adrido]

Vcpkg is a C++ Library Manager for Windows, Linux, and MacOS. https://github.com/Microsoft/vcpkg

It is really helpful because it makes it easy to manage Minetest's load of (optional) dependencies. Especially on Windows.

This PR changes all required things to make cmake+vcpkg work out of the box.
It also updates the documentation and adds instructions how to build on windows.

Here a checklist what's currently possible:

Triplets:
✔ x64-windows
❌ x64-windows-static (link error with irrlicht, openAL, curl and leveldb)
✔ x86-windows
❓ x86-windows-static (needs testing)

Required Features:
✔ build client
✔ build server
✔ irrlicht
✔ sqlite3
✔ zlib

Optional Features:
✔ curl
❓ curses (have no idea what's that)
✔ freetype
❓ gettext (supported by vcpkg but not tested)
❓ gles (probably not possible)
✔ leveldb
✔ luajit
✔ postgresql (changes to Minetest required)
❌ redis (Minetest depends on 'hiredis' which is Unix exclusive)
✔ sound
❓ spatial (there is a libary called libspatiallite)
❓ system-gmp (not supported by vcpkg)
❓ system-jsoncpp (supported by vcpkg but not tested)

It would be nice if someone could follow the documentation and find out if its complete and understandable.

That missing optional features can also added later, so I would consider this PR as ready for review.
This PR already includes #8295
Fixes #7642

Edit 14.03.2019: replaced the task list with "emotes" as it was not really a task list

[SmallJoker]

Should the old outdated windows build instructions kept or removed?

I think you can remove those. If someone still wants to compile the annoying way, then there's still the same guide on the wiki (maybe add a link to it?).

or schould it stay like it is (lua51.lib) this would make it impossible use it under Linux and OS-X

Guard it with ifs, to keep compatibility as much as possible.

[adrido]

Done. Regarding PostgreSQL:

minetest/src/CMakeLists.txt

Lines 161 to 186 in 77961aa

	if(ENABLE_POSTGRESQL)
	find_program(POSTGRESQL_CONFIG_EXECUTABLE pg_config DOC "pg_config")
	find_library(POSTGRESQL_LIBRARY pq)
	if(POSTGRESQL_CONFIG_EXECUTABLE)
	execute_process(COMMAND ${POSTGRESQL_CONFIG_EXECUTABLE} --includedir-server
	OUTPUT_VARIABLE POSTGRESQL_SERVER_INCLUDE_DIRS
	OUTPUT_STRIP_TRAILING_WHITESPACE)
	execute_process(COMMAND ${POSTGRESQL_CONFIG_EXECUTABLE}
	OUTPUT_VARIABLE POSTGRESQL_CLIENT_INCLUDE_DIRS
	OUTPUT_STRIP_TRAILING_WHITESPACE)
	# This variable is case sensitive for the cmake PostgreSQL module
	set(PostgreSQL_ADDITIONAL_SEARCH_PATHS ${POSTGRESQL_SERVER_INCLUDE_DIRS} ${POSTGRESQL_CLIENT_INCLUDE_DIRS})
	endif()
	find_package("PostgreSQL")
	if(POSTGRESQL_FOUND)
	set(USE_POSTGRESQL TRUE)
	message(STATUS "PostgreSQL backend enabled")
	# This variable is case sensitive, don't try to change it to POSTGRESQL_INCLUDE_DIR
	message(STATUS "PostgreSQL includes: ${PostgreSQL_INCLUDE_DIR}")
	include_directories(${PostgreSQL_INCLUDE_DIR})
	else()
	message(STATUS "PostgreSQL not found!")
	endif()
	endif(ENABLE_POSTGRESQL)

BTW is it intentional that
first find_library(POSTGRESQL_LIBRARY pq) is called
and then find_package("PostgreSQL") but at the end linking to POSTGRESQL_LIBRARY??? https://github.com/minetest/minetest/blob/master/src/CMakeLists.txt#L596

in case of vcpkg it looks like this:
find_library(POSTGRESQL_LIBRARY pq) --> POSTGRESQL_LIBRARY-NOTFOUND (its named libpq)
find_package("PostgreSQL") --> found --> sets PostgreSQL_LIBRARY to libpq.lib
target_link_libraries(${PROJECT_NAME} ${POSTGRESQL_LIBRARY}) --> ERROR

[adrido]

Rebased.

[adrido]

Rebased. As #8435 is merged, PostgreSQL is now working as well.

[ClobberXD]

Some minor grammar and spelling corrections in README :)

[SmallJoker]

Compiling on Windows sucks, so this is a good step forward. I tested this PR on Linux, to see whether the build system is still fine. It is, and Travis also confirms that.
👍

[rubenwardy]

This needs testing to make sure the buildbot script still works, and other ways of compiling on Windows too