I don''t know why this is needed. Most likely it's only a workaround
for DJGPP specific bug in DJGPP port of GNU make. make must normally
found g2c.hin also without that according VPATH, but seems that it 
doesn't happen (AP. 2004/01/03)

--- Makefile.in~1	2004-01-03 10:25:50.000000000 +0000
+++ Makefile.in	2004-01-03 10:24:56.000000000 +0000
@@ -190,7 +190,7 @@
 	cd $(srcdir) && autoconf
 
 # Dependence on Makefile serializes for parallel make.
-g2c.h: g2c.hin config.status Makefile
+g2c.h: $(top_srcdir)/g2c.hin config.status Makefile
 # Might try to avoid rebuilding everything if Makefile or configure
 # changes and g2c.h doesn't; see also the Makefile rule.  Should
 # depend on another stamp file rather than using the commented-out