installation requires GNU make

[bastistician]

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

writexl/src/Makevars

Lines 28 to 30 in 39efd5e

	# Base R already has this rule on Windows (but not Unix)
	%.a:
	@$(AR) crs $@ $^

are a GNU extension.

Why not define the recipe directly for the $(STATLIB) target, as in writexl 1.5.2 (which worked with bmake) ?

$(STATLIB): $(LIBXLSXWRITER)
	$(AR) -crs $@ $(LIBXLSXWRITER)

(NB: For maximum portability, the -s modifier should also be avoided; R-exts#Writing-portable-packages recommends the sequence rm, $(AR) -cr, $(RANLIB).)