bldchn, cosmetics: rename automake variable libtwsapi to twsapi
bldchn, require twsapi >= 0.4.0

It's not really required but twsapi.pc is only available since 0.4.0 and
libtwsapi.pc will be removed soon. We don't want to handle all this ...
bldchn, check for twsapi_config.h

This header was introduced in twsapi 0.4.2.
always use eConnect2, don't define TWSAPI_IPV6
Merge branch 'pkg-config-twsapi'

* pkg-config-twsapi:
  always use eConnect2, don't define TWSAPI_IPV6
  bldchn, check for twsapi_config.h
  bldchn, require twsapi >= 0.4.0
  bldchn, cosmetics: rename automake variable libtwsapi to twsapi
twsapi 0.5, TagValueList moves up to namespace IB
twsapi 0.5, Contract::comboLegs is a shared pointer
implement missing callback for twsapi >= 0.4

The marketDataType() callback was introduced already in twsapi 0.4 but
became pure virtual only in 0.5.
twsapi 0.5.0, implement missing callback
Merge branch 'twsapi-0.5'

* twsapi-0.5:
  twsapi 0.5.0, implement missing callback
  implement missing callback for twsapi >= 0.4
  twsapi 0.5, Contract::comboLegs is a shared pointer
  twsapi 0.5, TagValueList moves up to namespace IB