installation requires GNU make

When using BSD (or POSIX) implementations of `make`, e.g., `MAKE=bmake`, package installation fails with ``` /usr/bin/ld: cannot find -lstatxlsxwriter: No such file or directory ``` This is because pattern rules without prerequisites as used in <https://github.com/ropensci/writexl/blob/39efd5e244e52d3ae8ebeea810c312fde7a29334/src/Makevars#L28-L30> are a GNU extension. Why not define the recipe directly for the `$(STATLIB)` target, as in writexl 1.5.2 (which worked with `bmake`) ? ```Makefile $(STATLIB): $(LIBXLSXWRITER) $(AR) -crs $@ $(LIBXLSXWRITER) ``` (NB: For maximum portability, the `-s` modifier should also be avoided; [R-exts#Writing-portable-packages](https://cran.r-project.org/doc/manuals/r-devel/R-exts.html#Writing-portable-packages) recommends the sequence `rm`, `$(AR) -cr`, `$(RANLIB)`.)