Add Header Library support to Make

We currently use static libraries without any source files to represent
header libraries, but Soong actually has cc_library_headers. So to
export those in a separate namespace from static libraries, implement
them in Make as well.

This also adds a nice pretty-warning / pretty-error macro that can be
used to print out standard warning messages pointing to the real source
file having the problem.

Test: Use a header library exported by Soong in a Make module
Change-Id: I3486539e247524cb82a20620745fc7be03014e14
diff --git a/core/config.mk b/core/config.mk
index 2f43f46..3fd7cc2 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -92,6 +92,7 @@
 BUILD_HOST_STATIC_LIBRARY:= $(BUILD_SYSTEM)/host_static_library.mk
 BUILD_HOST_SHARED_LIBRARY:= $(BUILD_SYSTEM)/host_shared_library.mk
 BUILD_STATIC_LIBRARY:= $(BUILD_SYSTEM)/static_library.mk
+BUILD_HEADER_LIBRARY:= $(BUILD_SYSTEM)/header_library.mk
 BUILD_AUX_STATIC_LIBRARY:= $(BUILD_SYSTEM)/aux_static_library.mk
 BUILD_AUX_EXECUTABLE:= $(BUILD_SYSTEM)/aux_executable.mk
 BUILD_SHARED_LIBRARY:= $(BUILD_SYSTEM)/shared_library.mk