Warn the user is either flex or bison is missing.

git-svn-id: http://lm-sensors.org/svn/lm-sensors/trunk@5869 7894878c-1315-0410-8ee3-d5d059ff63e0

CHANGES

@@ -2,6 +2,7 @@ lm-sensors CHANGES file
 -----------------------
 
 SVN HEAD
+  Makefile: Check for bison and flex
   sensors-detect: Improve LM90 and W83L771 detection
 
 3.2.0 (2010-10-10)

Makefile

@@ -266,7 +266,11 @@ manhtml:
 
 
 # Flex and Bison
-%c: %y
+%.c: %.y
+	@if ! which $(BISON) 2> /dev/null ; then \
+		echo "Please install $(BISON), then run \"make clean\" and try again" ; \
+		false ; \
+	fi
 	$(BISON) -p sensors_yy -d $< -o $@
 
 ifeq ($(DEBUG),1)
@@ -276,4 +280,8 @@ FLEX_FLAGS := -Psensors_yy -t -Cfe -8
 endif
 
 %.c: %.l
+	@if ! which $(FLEX) 2> /dev/null ; then \
+		echo "Please install $(FLEX), then run \"make clean\" and try again" ; \
+		false ; \
+	fi
 	$(FLEX) $(FLEX_FLAGS) $< > $@