depends: Propagate only specific CLI variables to sub-makes

We want to supply well-known vars to ./configure scripts to do with as
they please. However, we do _not_ want to override these well-known vars
at make-time as certain build systems expect a self-mangled version of
these well-known vars.

For example, freetype and bdb will prepend `libtool --mode=compile' to
CC and CXX, which, if we override CC on the command line at make-time,
will break the build.

depends/Makefile

@@ -4,6 +4,30 @@
 print-%:
 	@echo $* = $($*)
 
+# When invoking a sub-make, keep only the command line variable definitions
+# matching the pattern in the filter function.
+#
+# e.g. invoking:
+#   $ make A=1 C=1 print-MAKEOVERRIDES print-MAKEFLAGS
+#
+# with the following in the Makefile:
+#   MAKEOVERRIDES := $(filter A=% B=%,$(MAKEOVERRIDES))
+#
+# will print:
+#   MAKEOVERRIDES = A=1
+#   MAKEFLAGS = -- A=1
+#
+# this is because as the GNU make manual says:
+#   The command line variable definitions really appear in the variable
+#   MAKEOVERRIDES, and MAKEFLAGS contains a reference to this variable.
+#
+# and since the GNU make manual also says:
+#   variables defined on the command line are passed to the sub-make through
+#   MAKEFLAGS
+#
+# this means that sub-makes will be invoked as if:
+#   $(MAKE) A=1 blah blah
+MAKEOVERRIDES := $(filter V=%,$(MAKEOVERRIDES))
 SOURCES_PATH ?= $(BASEDIR)/sources
 BASE_CACHE ?= $(BASEDIR)/built
 SDK_PATH ?= $(BASEDIR)/SDKs