steam-api: compare API return values ignoring case
configure: added an argument for debugging features

Debugging features:
  - Enable all compiler warnings
  - Enable compiler debugging
  - Enable verbose HTTP replies
configure: ensure the compiler uses the C99 standard

The C99 standard is needed for the usage of designated initializers.

This is done by making use of the AC_PROG_CC_C99 macro which is present
in autoconf >= 2.59. On systems with older autoconf versions, this macro
will not be called.
steam-api: fixed the handling of "Not Logged On" errors

Currently, the implementation for checking for the error is broken due
to the the steam_api_relogon_check function. Additionally, the errors
were not being handled upon a poll reply.
steam-api: fixed UMQID not being checked at logon