Visual Studio linker problem (strtoll)

I am maintaining a legacy application built with Fox 1.6, and in the 
course of that work, have stumbled upon a problem when building with 
Visual Studio 2019 that I've isolated into a small sample. The linker 
message in question is this:

Build started...
1>------ Build started: Project: strtoll_bug, Configuration: Debug Win32 
------
1>ucrt.lib(api-ms-win-crt-convert-l1-1-0.dll) : error LNK2005: _strtoll 
already defined in FOX-1.6.lib(strtoll.obj)
1>   Creating library X:\Debug\strtoll_bug.lib and object 
X:\Debug\strtoll_bug.exp
1>X:\Debug\strtoll_bug.exe : fatal error LNK1169: one or more multiply 
defined symbols found
1>Done building project "strtoll_bug.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

This is the source (main.cpp):

#include <cassert>
#include <cstdlib>

#include <fx.h>
#include <FXString.h>

int main(void)
{
	const char* input = "1234567890test";
	char* endp;
	long long value = strtoll(input, &endp, 10);
	assert(value == 1234567890);
	assert(endp[0] == 't');
	value = FXLongVal(FXString(input), 10);
	assert(value == 1234567890);
	return 0;
}

It appears that FOX-1.6.lib defines a strtoll function that conflicts 
with the one in the runtime? Why is that?