Use gendef instead of pexports for building windows .def files

Modern msys systems lack pexports but have gendef instead, so use that.

Discussion: https://postgr.es/m/3ccde7a9-e4f9-e194-30e0-0936e6ad68ba@dunslane.net

Backpatch to release 9.4 to enable building with perl on older branches.
Before that pexports is not used for plperl.

src/pl/plperl/GNUmakefile

@@ -48,7 +48,7 @@ lib$(perlwithver).a: $(perlwithver).def
 	dlltool --dllname $(perlwithver).dll --def $(perlwithver).def --output-lib lib$(perlwithver).a
 
 $(perlwithver).def: $(PERLDLL)
-	pexports $^ > $@
+	gendef - $^ > $@
 
 endif # win32
 

src/pl/plpython/Makefile

@@ -69,7 +69,7 @@ libpython${pytverstr}.a: python${pytverstr}.def
 	dlltool --dllname python${pytverstr}.dll --def python${pytverstr}.def --output-lib libpython${pytverstr}.a
 
 python${pytverstr}.def:
-	pexports $(PYTHONDLL) > $@
+	gendef - $(PYTHONDLL) > $@
 
 endif # win32
 

src/pl/tcl/Makefile

@@ -46,7 +46,7 @@ lib$(tclwithver).a: $(tclwithver).def
 	dlltool --dllname $(tclwithver).dll --def $(tclwithver).def --output-lib lib$(tclwithver).a
 
 $(tclwithver).def: $(TCLDLL)
-	pexports $^ > $@
+	gendef - $^ > $@
 
 endif # win32