Otherwise the compiler may optimize out the reference to the symbol as the
previous version was not really using this. This leads to symbols that are
only in a header but not in the given libraries to be reported as present.

This came up on the first try to fix bug 11333 as "gcc -O3" would optimize
out the reference to pthread_create() so the correct library the symbol is in
was not detected.

The new test code was suggested by Brad King.