Makefile.mk: improve a GNU Make hack [ci skip]

Replace the hack of using `$() ` to represent a single space. The new
method silences the `--warn-undefined-variables` debug warning and it's
also a better-known form of solving this problem.

Reviewed-by: Jay Satiro
Closes #10031

lib/Makefile.mk

@@ -324,7 +324,7 @@ ifdef WIN32
   _LIBS += -lws2_32 -lcrypt32 -lbcrypt
 endif
 
-ifneq ($(findstring 11,$(subst $() ,,$(SSLLIBS))),)
+ifneq ($(findstring 11,$(subst $(subst ,, ),,$(SSLLIBS))),)
   CPPFLAGS += -DCURL_WITH_MULTI_SSL
 endif