Build: allow overriding DEPS_BUILD_DIR and DEPS_INSTALL_DIR for make deps.

GNUmakefile

@@ -45,13 +45,19 @@ endif
 
 # Dependencies DIR's
 DEPS_SOURCE_DIR:=$(BLENDER_DIR)/build_files/build_environment
+
+ifndef DEPS_BUILD_DIR
 	DEPS_BUILD_DIR:=$(BUILD_DIR)/deps
+endif
+
+ifndef DEPS_INSTALL_DIR
 	DEPS_INSTALL_DIR:=$(shell dirname "$(BLENDER_DIR)")/lib/$(OS_NCASE)
 
 	ifneq ($(OS_NCASE),darwin)
 		# Add processor type to directory name
 		DEPS_INSTALL_DIR:=$(DEPS_INSTALL_DIR)_$(shell uname -p)
 	endif
+endif
 
 # Allow to use alternative binary (pypy3, etc)
 ifndef PYTHON