Make autoconf-based build quiet by default.

Add V=1 to the make command line will show the full
commands but by default we just show the tool-type
and the output file.  This is generally muche easier
on the eye and makes warnings and errors more clearly
visible.
Fix compiler warnings in Native Client and Linux builds.
Update Native Client build script and test Makefile.

The change here are not to export SOURCES while building
SDL itself (otherwise the test sources get built into
libSDL).  Also add include/pnacl to the include path so
that <sys/mount.h> can be found at compile time.