Makefile: add [POSIX | WIN32]_THREADS variable

Makefile

@@ -570,6 +570,7 @@ build-only-$(1)_$(3): PKG = $(1)
 build-only-$(1)_$(3): TARGET = $(3)
 build-only-$(1)_$(3): BUILD_$(if $(findstring shared,$(3)),SHARED,STATIC) = TRUE
 build-only-$(1)_$(3): BUILD_$(if $(call seq,$(TARGET),$(BUILD)),NATIVE,CROSS) = TRUE
+build-only-$(1)_$(3): $(if $(findstring posix,$(TARGET)),POSIX,WIN32)_THREADS = TRUE
 build-only-$(1)_$(3): LIB_SUFFIX = $(if $(findstring shared,$(3)),dll,a)
 build-only-$(1)_$(3): BITS = $(if $(findstring x86_64,$(3)),64,32)
 build-only-$(1)_$(3): BUILD_TYPE = $(if $(findstring debug,$(3) $($(1)_CONFIGURE_OPTS)),debug,release)