make: resolve LDFLAGS based on system

makefile

@@ -1,7 +1,10 @@
+.POSIX:
+
 CC ?= gcc
 CFLAGS = -Wall -pedantic --std=c99 -O2
 DBG_CFLAGS = -Wall -Werror -pedantic --std=c99 -O0 -g
-LDFLAGS != if [ "$(uname)" = 'Linux' ]; then printf '-lcrypt'; fi
+SYSTEM != uname
+LDFLAGS != if [ '${SYSTEM}' = 'Linux' ]; then echo '-lcrypt'; fi
 PREFIX = /usr/local
 MANPREFIX = ${PREFIX}/share/man