Explicitly set the `make` compiler to `cc`.

This makes it work out of the box on GNU systems where GNU make defaults
to c99, which doesn't accept -std=c89.

configure

@@ -195,6 +195,7 @@ cat << EOF > Makefile
 # Generated by '$0' on $(date).
 # This file should generally not be manually edited.
 
+CC = cc
 PREFIX = ${PREFIX}
 CFLAGS = ${CFLAGS}
 LDFLAGS = ${LDFLAGS}