Fix build w/ rtools4

Author: jeroen

src/Makevars.win

@@ -13,8 +13,9 @@ PKG_LIBS = -L$(LIBDIR) -lmyrustlib -lws2_32 -ladvapi32 -luserenv -ldbghelp -lbcr
 ifeq ($(R_COMPILED_BY),gcc 8.3.0)
 GCCPATH=$(subst C:\,/c/,$(RTOOLS40_HOME))/mingw$(WIN)/bin
 else
+# Either rtools40 or rtools42 ucrt toolchain is OK
+GCCPATH=$(subst C:\,/c/,$(RTOOLS40_HOME))/ucrt64/bin:$(R_RTOOLS42_PATH)
 export CARGO_TARGET_X86_64_PC_WINDOWS_GNU_LINKER=gcc
-GCCPATH=$(subst C:\,/c/,$(RTOOLS42_HOME))/x86_64-w64-mingw32.static.posix/bin
 endif
 
 all: clean rustup