Add: introduce CMake for project management

CMake works on all our supported platforms, like MSVC, Mingw, GCC,
Clang, and many more. It allows for a single way of doing things,
so no longer we need shell scripts and vbs scripts to work on all
our supported platforms.

Additionally, CMake allows to generate project files for like MSVC,
KDevelop, etc.

This heavily reduces the lines of code we need to support multiple
platforms from a project perspective.

Addtiionally, this heavily improves our detection of libraries, etc.

cmake/LinkPackage.cmake

@@ -0,0 +1,18 @@
+function(link_package NAME)
+    cmake_parse_arguments(LP "ENCOURAGED" "TARGET" "" ${ARGN})
+
+    if (${NAME}_FOUND)
+        string(TOUPPER "${NAME}" UCNAME)
+        add_definitions(-DWITH_${UCNAME})
+        if (LP_TARGET AND TARGET ${LP_TARGET})
+            target_link_libraries(openttd ${LP_TARGET})
+            message(STATUS "${NAME} found -- -DWITH_${UCNAME} -- ${LP_TARGET}")
+        else()
+            include_directories(${${NAME}_INCLUDE_DIRS} ${${NAME}_INCLUDE_DIR})
+            target_link_libraries(openttd ${${NAME}_LIBRARIES} ${${NAME}_LIBRARY})
+            message(STATUS "${NAME} found -- -DWITH_${UCNAME} -- ${${NAME}_INCLUDE_DIRS} ${${NAME}_INCLUDE_DIR} -- ${${NAME}_LIBRARIES} ${${NAME}_LIBRARY}")
+        endif()
+    elseif (LP_ENCOURAGED)
+        message(WARNING "${NAME} not found; compiling OpenTTD without ${NAME} is strongly disencouraged")
+    endif()
+endfunction()