GNUmakefile: fix installation logic for libstdbuf

It is better to check whether we're actually compiling libstdbuf, rather than to check whether
we're on windows, in order to decide whether libstdbuf should get installed.

Signed-off-by: Etienne Cordonnier <ecordonnier@snap.com>

Author: Ecordonnier

GNUmakefile

@@ -475,7 +475,7 @@ endif
 
 install: build install-manpages install-completions install-locales
 	mkdir -p $(INSTALLDIR_BIN)
-ifneq ($(OS),Windows_NT)
+ifneq (,$(and $(findstring stdbuf,$(UTILS)),$(findstring feat_external_libstdbuf,$(CARGOFLAGS))))
 	mkdir -p $(DESTDIR)$(LIBSTDBUF_DIR)
 	$(INSTALL) -m 755 $(BUILDDIR)/deps/libstdbuf* $(DESTDIR)$(LIBSTDBUF_DIR)/
 endif