Running multiple consecutive builds on any target that depends on
secp256k1 will trigger a rebuild of the library once every 2 calls.
The reason for this is that the gen_context utility is producing the
ecmult_static_context.h file under the src directory, but the custom
command expects the output in the current binary directory which is a
level above. The custom command ends up running everytime to try
regenerating the output, which in turns cause secp256k1 to rebuild as
the header is modified.