Add: support for emscripten (play-OpenTTD-in-the-browser)

Emscripten compiles to WASM, which can be loaded via
HTML / JavaScript. This allows you to play OpenTTD inside a
browser.

Co-authored-by: milek7 <me@milek7.pl>

os/emscripten/cmake/FindLibLZMA.cmake

@@ -0,0 +1,20 @@
+# LibLZMA is a recent addition to the emscripten SDK, so it is possible
+# someone hasn't updated his SDK yet. Test out if the SDK supports LibLZMA.
+include(CheckCXXSourceCompiles)
+set(CMAKE_REQUIRED_FLAGS "-sUSE_LIBLZMA=1")
+
+check_cxx_source_compiles("
+    #include <lzma.h>
+    int main() { return 0; }"
+    LIBLZMA_FOUND
+)
+
+if (LIBLZMA_FOUND)
+        add_library(LibLZMA::LibLZMA INTERFACE IMPORTED)
+        set_target_properties(LibLZMA::LibLZMA PROPERTIES
+                INTERFACE_COMPILE_OPTIONS "-sUSE_LIBLZMA=1"
+                INTERFACE_LINK_LIBRARIES "-sUSE_LIBLZMA=1"
+        )
+else()
+        message(WARNING "You are using an emscripten SDK without LibLZMA support. Many savegames won't be able to load in OpenTTD. Please apply 'emsdk-liblzma.patch' to your local emsdk installation.")
+endif()

os/emscripten/cmake/FindPNG.cmake

@@ -0,0 +1,7 @@
+add_library(PNG::PNG INTERFACE IMPORTED)
+set_target_properties(PNG::PNG PROPERTIES
+        INTERFACE_COMPILE_OPTIONS "-sUSE_LIBPNG=1"
+        INTERFACE_LINK_LIBRARIES "-sUSE_LIBPNG=1"
+)
+
+set(PNG_FOUND on)

os/emscripten/cmake/FindSDL2.cmake

@@ -0,0 +1,7 @@
+add_library(SDL2::SDL2 INTERFACE IMPORTED)
+set_target_properties(SDL2::SDL2 PROPERTIES
+        INTERFACE_COMPILE_OPTIONS "-sUSE_SDL=2"
+        INTERFACE_LINK_LIBRARIES "-sUSE_SDL=2"
+)
+
+set(SDL2_FOUND on)

os/emscripten/cmake/FindZLIB.cmake

@@ -0,0 +1,7 @@
+add_library(ZLIB::ZLIB INTERFACE IMPORTED)
+set_target_properties(ZLIB::ZLIB PROPERTIES
+        INTERFACE_COMPILE_OPTIONS "-sUSE_ZLIB=1"
+        INTERFACE_LINK_LIBRARIES "-sUSE_ZLIB=1"
+)
+
+set(ZLIB_FOUND on)