20231116.00

build: fix DESTDIR concatenation (#124)

When updating the COS build we found out that the build system wasn't
properly concatenating DESTDIR and the target dirs if DESTDIR didn't
end with a slash. The following make call would produce an invalid
result:

make DESTDIR=/path/to/target LIBDIR=lib64

One would expect to have lib artifacts installed to
/path/to/target/lib64 instead we were installing artifacts to
/path/to/targetlib64.