Clean up use of function wrapping
Currently, building BIND using "--without-dlopen" universally breaks building unit tests which employ the --wrap linker option (because the replacement functions are put in a shared library and building shared objects requires "--with-dlopen"). Fix by moving the overridden symbol, isc_nmhandle_unref(), to lib/ns/tests/nstest.c and dropping lib/ns/tests/wrap.c altogether. This simplifies Makefiles a lot and prevents --without-dlopen from messing with the process of building unit tests.
Remove related bits of code from configure.ac which are made redundant by the above changes.
Put the replacement definition of isc_nmhandle_unref() inside an #ifdef block, so that the build does not break for non-libtool builds (see below).
These changes allow the broadest possible set of build variants to work while also simplifying the build process:
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for libtool builds, overriding isc_nmhandle_unref() is done by placing that symbol directly in lib/ns/tests/nstest.c and relying on the dynamic linker to perform symbol resolution properly when the test binary is run,
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for non-libtool builds, overriding isc_nmhandle_unref() is done using the --wrap linker option (the libtool approach cannot be used in this case as multiple strong symbols with the same name cannot coexist in the same binary),
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the "--without-dlopen" option no longer affects building unit tests.
Closes #1725 (closed)