essentially as in kernel makefiles, so that module sources can include
<stddef.h> and other standard headers. Only add the second path when
the first path can't be found, instead of when DESTDIR is defined.
Adding it used to be just an obfuscation.
Use "${.OBJDIR}" instyead of "." in -I paths. Using "${.OBJDIR}" just
gave more verbose command lines and depend files.
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