meson: make nm binary optional

meson: make nm binary optional

This makes nm not required, but used if found. In general I imagine that
this means that on windows nm wont be found, and on other platforms it
will.

v2: - fix gbm and egl symbols check tests to only be run if nm is found
    - reword commit message to reflect the code change

Reviewed-by: Eric Anholt <eric@anholt.net>