Since Autoconf places some important feature flags only into the
configuration header, it is necessary to include it globally to
guarantee a consistent build.
A few of these were pretty serious, like missing arguments,
passing integers where a string was expected, and so on.
In some places, change the types used by the code rather than
just the format strings.
Make vxi.h the first #include in all affected files and #undef the
_POSIX_C_SOURCE macro in vxi.h.
This avoids various build issues on e.g. FreeBSD or Mac OS X where
setting _POSIX_C_SOURCE leads to the unavailability of certain types
such as u_long (as used in the VXI/RPC code).
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.