Revise handling of version.h to avoid build problems.

A patch to insert the version string from git broke compilation when
the source was not a git repository or when building in a read-only
source tree.  This patch avoids breaking compilation by using a
graceful failure mechanism for generating the version string and does
not write to the source tree.

Makefile.in

@@ -182,10 +182,23 @@ iverilog-vpi.ps: $(srcdir)/iverilog-vpi.man
 iverilog-vpi.pdf: iverilog-vpi.ps
 	ps2pdf iverilog-vpi.ps iverilog-vpi.pdf
 
-version.h: .git/index
-	git --git-dir $(srcdir)/.git describe \
-	    | sed -e 's;\(.*\);#define VERSION_TAG "\1";' > $@tmp
-	diff $@tmp $(srcdir)/$@ > /dev/null 2>&1 || mv $@tmp $(srcdir)/$@
+# For VERSION_TAG in driver/main.c, first try git-describe, then look for a
+# version.h file in the source tree (included in snapshots and releases), and
+# finally use nothing.
+.PHONY: version.h
+version.h:
+	@if test -d $(srcdir)/.git; then \
+	    echo "Using git-describe for VERSION_TAG"; \
+	    git --git-dir $(srcdir)/.git describe \
+		| sed -e 's;\(.*\);#define VERSION_TAG "\1";' > $@tmp; \
+	    diff $@tmp $@ > /dev/null 2>&1 || mv $@tmp $@; \
+	elif test -r $(srcdir)/$@; then \
+	    echo "Using $(srcdir)/$@ for VERSION_TAG"; \
+	    diff $(srcdir)/$@ $@ > /dev/null 2>&1 || cp $(srcdir)/$@ $@; \
+	else \
+	    echo "Using empty VERSION_TAG"; \
+	    echo '#define VERSION_TAG ""' > $@; \
+	fi
 
 ifeq (@MING32@,yes)
 INSTALL_DOC = $(prefix)/iverilog-vpi.pdf $(mandir)/man1/iverilog-vpi.1