This makes the code shorter, simpler and more consistent, and also
ensures that the (same) debug messages are always emitted and the
packet.payload field is consistently set to NULL always, etc.
SR_CONF_NUM_LOGIC_CHANNELS is defined as SR_T_INT32. Create the
GVariant with the correct type to avoid a type mismatch error in
sr_variant_type_check().
Since Autoconf places some important feature flags only into the
configuration header, it is necessary to include it globally to
guarantee a consistent build.
Move the include flags for files in the source tree from
configure.ac to Makefile.am where they belong. Also use
AM_CPPFLAGS instead of CFLAGS/CXXFLAGS to make sure the
files in the build/source tree are always picked up first.
Also, remove the include/libsigrok sub-directory from the
search path, thereby making the <libsigrok/> prefix mandatory
when building libsigrok itself. This matches the convention
already imposed on users of the library.