build: strip: unify win32 and use the configured $STRIP

- There's no need to force STRIP_BINARIES on windows since --enable-strip (at
  configure) already does exactly that, if one wants to.
- Use the contigured $STRIP instead of the native 'strip', useful when
  cross building tcc.
- 'make install-strip' now also strips libtcc.dll on windows (it already does
  so now with --enable-strip, and previously it always stripped it).

Summary of current strip options for all platforms:
- configure --enable strip -> 'install -s' for the binaries.
- make install-strip: installs and then configured $STRIP the binaries.
- Otherwise -> no stripping.

Makefile

@@ -24,7 +24,6 @@ LINK_LIBTCC =
 LIBS =
 
 ifdef CONFIG_WIN32
- STRIP_BINARIES = yes
  LIBTCC = libtcc.dll
 else
  LIBS=-lm
@@ -228,7 +227,10 @@ else
 endif
 
 install-strip: install
-	strip $(foreach PROG,$(PROGS),"$(bindir)"/$(PROG))
+	$(STRIP) $(foreach PROG,$(PROGS),"$(bindir)"/$(PROG))
+ifdef CONFIG_WIN32
+	$(STRIP) "$(bindir)/$(LIBTCC)"
+endif
 
 ifndef CONFIG_WIN32
 install: $(PROGS) $(TCCLIBS) $(TCCDOCS)