Use patch.patch() when checking for patch availability. The test is closer to what we want to know. It also makes sense when you deal with patched versions of patch() as most people do under win32.

CC := gcc
CFLAGS := -g -O2 -Wall -Werror

prefix ?= /usr/bin

hgsh: hgsh.o
	$(CC) -o $@ $<

install: hgsh
	install -m755 hgsh $(prefix)

clean:
	rm -f *.o hgsh