Pkg 'fs' 1.3.0 from source fails link step under RStudio 1.2.1335 & RTools

Platform Windows Pro x64
Scenario: Refresh of R & RStudio environment after a period of dis-use this particular computer. Update R to 3.6.0 from CRAN, update RStudio to 1.2.1335. For R 64 bit only components installed, for RTools both 32 & 64 bit components installed.

Used RStudio UI to mass update all packages. Binary version of 'fs' is 1.2.7 while source is 1.3.0, so selected source. This reporter is a relative R n00b but has been hacking build tool-chains, make & C for decades, so I have some idea of what I'm looking at. RS Console shows compile proceeding no fatal errors of individual .o though two libuv sub-components of 'fs' reports two code warnings. Sub-component libuv.a file created no error. Then final link step to create the pre-install fs.dll file fails with 500+ undefined symbols. The linker command line is

C:/Rtools/3.5/mingw_64/bin/g++ -shared -s -static-libgcc -o fs.dll tmp.def error.o dir.o fs.o link.o path.o file.o utils.o id.o windows/getmode.o RcppExports.o ./libuv/libuv.a -lpthread -lws2_32 -lkernel32 -lpsapi -liphlpapi -lshell32 -luserenv -LC:/PROGRA~1/R/R-36~1.X/bin/x64 -lR

Re-installed R and RTools and no joy.

Examined source tree fs_1.3.0/fs/src/libuv/Makefile.mingw and all .o components listed as part of libuv appear to be accounted for in the log of building libuv.a. This suggests that the final link command line constructed by make from fs_1.3.0/fs/src/ in the package is missing one or more library specs for a Windows build, but which? Can't find link libs pthread, kernel32, psapi, iphlpapi, shell32 or userenv in the RTools tree. psapi.dll, kernel32.dll, iphlpapi.dll, shell32.dll and userenv.dll show buried in C:/Windows

On the possibility that there is something funky with my install of R & RTools, has anyone else encountered errors building package fs 1.3.0 via RStudio and RTools?