Tweak PWD in the Makefile to be more specific

If the Makefile is started with "make -C mythtv" from a Makefile another level
up, $PWD is still seeming to be set from the top level Makefile.  This forces
a reset of $PWD from the shell at this level, causing version.sh to put
version.cpp in the correct place.

mythtv/Makefile

@@ -26,6 +26,10 @@ all: version.cpp subdirs
 config.mak:
 	$(error run configure to create $@)
 
+# Override PWD in case this is run from a makefile at a higher level with
+# make -C mythtv
+PWD := $(shell pwd)
+
 version.cpp:	libs/libmythdb/mythversion.h
 	sh version.sh $(PWD)