Escaping quotes in CFLAGS

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

Building print/luatex fails (at least with clang) because of the following 
definition in Makefile:
CFLAGS+=        -DLUA_ROOT=\"${PREFIX}/\"
changing that to
CFLAGS+=        -DLUA_ROOT='"${PREFIX}/"'
seems to work but I'm unsure whether this is the right fix.