Post scriptum: If -Werror is removed, the build runs through, as seen here: https://travis-ci.org/inexorgame/entity-system/jobs/502578790

I get this too.

vcpkg/buildtrees/crossguid/src/365d430337-f320045b03.clean/src/guid.cpp:403:7: error: unused function 'swap' [-Werror,-Wunused-function]
void swap(xg::Guid &lhs, xg::Guid &rhs)

The -Werror option should be removed from the CMakeLists.txt or a way to disable it in case. An unused function warning should never be treated as an error! (swap function causing the build to fail!)
Replace in CMakeLists.txt with those lines.
if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
set(WARNINGS "-Wall")
elseif(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
set(WARNINGS "-Wall")
elseif(MSVC)
set(WARNINGS "/WX" "/W4")
endif()

Currently, it makes it impossible to include this project using FetchContent_Declare(crossguid GIT_REPOSITORY https://github.com/graeme-hill/crossguid.git) when compiling for anything else than Windows (unfortunately other systems exist :-))
The only remaining way is to get a copy of the code without pulling it from Git.