diff --git a/Makefile b/Makefile
index 418b564..fd15366 100644
--- a/Makefile
+++ b/Makefile
@@ -18,7 +18,7 @@ PREFIX ?= /usr/local
 
 # normalized prefix
 PREFIX_N != dirname $(PREFIX)/.
-MANDIR != [ $(PREFIX_N) = / ] && printf '/usr/share/man\n' \
+MANDIR != test $(PREFIX_N) -eq / && printf '/usr/share/man\n' \
 	|| printf '/share/man\n'
 SYSEXITS != printf '\043include <sysexits.h>\n' | cpp -M - | tr ' ' '\n' \
 	| sed -n 's/sysexits\.h//p' || printf 'include\n'