Creating a shell if structure in Makefile

Did you ever wonder how to make a shell if structure in a Makefile?

Suppose the following (taken from the Open Source project Make CD-ROM Recovery):

If the local configuration file /etc/mkcdrec.conf exists then check some local variable first instead of the global configuration file Config.sh.

Use the following if construction in the Makefile to accomplish this:

ISOFS_DIR := $(shell if  grep -q ISOFS_DIR= /etc/mkcdrec.conf 2>/dev/null; then \
   grep ISOFS_DIR= /etc/mkcdrec.conf | grep -v ^\# | cut -d= -f 2; \
else \
   grep ISOFS_DIR= Config.sh | grep -v ^\# | cut -d= -f 2; \
fi)

The important rule is that all commands should be on one line (or continued with \).