CMakeLists: Use PROJECT_SOURCE_DIR instead of CMAKE_SOURCE_DIR

This is more localized to what we want to enforce directory-wise with
the project. CMAKE_SOURCE_DIR indicates the root of the source tree, but
this would cause the wrong behavior if someone included yuzu as part of
a larger buildsystem (for whatever reason). Instead, we want to use the
directory where the "project(yuzu)" command was declared as the root
path reference.

src/web_service/CMakeLists.txt

@@ -10,7 +10,7 @@ add_library(web_service STATIC
 create_target_directory_groups(web_service)
 
 get_directory_property(OPENSSL_LIBS
-        DIRECTORY ${CMAKE_SOURCE_DIR}/externals/libressl
+        DIRECTORY ${PROJECT_SOURCE_DIR}/externals/libressl
         DEFINITION OPENSSL_LIBS)
 target_compile_definitions(web_service PUBLIC -DCPPHTTPLIB_OPENSSL_SUPPORT)
 target_link_libraries(web_service PRIVATE common json-headers ${OPENSSL_LIBS} httplib lurlparser)