Minor rework of how driver-vpi is supported.

Add driver-vpi to either SUBDIRS or NOTUSED depending on if we
are building for MinGW or not. This is more logical and allows
the clean and distclean targets to be simplified.
(cherry picked from commit 145133bdb0)

Makefile.in

@@ -43,6 +43,12 @@ SUBDIRS = ivlpp vvp vpi libveriuser cadpli tgt-null tgt-stub tgt-vvp \
 # Only run distclean for these directories.
 NOTUSED = tgt-fpga tgt-pal tgt-verilog
 
+ifeq (@MINGW32@,yes)
+SUBDIRS += driver-vpi
+else
+NOTUSED += driver-vpi
+endif
+
 # To get the version headers to build correctly we only want to look
 # for C++ files in the source directory. All other files will require
 # an explicit $(srcdir).
@@ -96,9 +102,6 @@ check: all
 
 clean:
 	$(foreach dir,$(SUBDIRS),$(MAKE) -C $(dir) $@ && ) true
-ifeq (@MINGW32@,no)
-	(cd driver-vpi ; $(MAKE) $@)
-endif
 	rm -f *.o parse.cc parse.h lexor.cc
 	rm -f ivl.exp iverilog-vpi.man iverilog-vpi.pdf iverilog-vpi.ps parse.output
 	rm -f syn-rules.output dosify.exe ivl@EXEEXT@ check.vvp
@@ -108,9 +111,6 @@ endif
 
 distclean: clean
 	$(foreach dir,$(SUBDIRS),$(MAKE) -C $(dir) $@ && ) true
-ifeq (@MINGW32@,no)
-	(cd driver-vpi ; $(MAKE) $@)
-endif
 	$(foreach dir,$(NOTUSED),$(MAKE) -C $(dir) $@ && ) true
 	rm -f Makefile config.status config.log config.cache
 	rm -f stamp-config-h config.h
@@ -177,9 +177,7 @@ ivl@EXEEXT@: $O
 	$(CXX) $(LDFLAGS) -o ivl@EXEEXT@ $O $(dllib)
 endif
 
-ifeq (@MINGW32@,yes)
-SUBDIRS += driver-vpi
-else
+ifeq (@MINGW32@,no)
 all: dep iverilog-vpi
 
 iverilog-vpi: $(srcdir)/iverilog-vpi.sh Makefile