fix: turn off libxml2's new thread local storage feature

Using thread-local variables in position-independent code (i.e.,
combining `__thread` variables with the `-fPIC` compiler option)
introduces a (non-weak) reference to `__tls_get_addr` which resolves
to `ld-linux` on glibc systems and `libwinpthread` on windows systems.

This is an issue when precompiling for musl on glibc systems. If and
when we can build a separate musl native gem, we can revisit this
decision.

ext/nokogiri/extconf.rb

@@ -919,6 +919,7 @@ def configure
       "--with-c14n",
       "--with-debug",
       "--with-threads",
+      "--without-tls", # see https://github.com/sparklemotion/nokogiri/issues/3031
       "CPPFLAGS=#{cppflags}",
       "CFLAGS=#{cflags}",
     ]