configure.ac: use AC_LANG_SOURCE where appropriate
configure.ac: fix some autoconf warnings
configure.ac: enable silent build rules

Silent build rules make it much less likely for warnings to go by
unnoticed.
configure.ac: fix INLINE definition

The inline attribute must still be prefixed with 'inline' to work
correctly.
jcmaster.c: fix compiler warnings

* remove unused variable size[8]
* initialize base_scan_idx
turbojpeg.c: clean up warnings around getinstance

This macro defined two local variables, cinfo and dinfo, but both
aren't always used.  Add a (void)cinfo; and (void)dinfo in there
to hush up the compiler.
tjbench.c: remove unused retvals from void func

There was a macro defining retval which was being invoked from void
functions.  The reval was likely stale code because it was not used.
adjust_exif_parameters only needed in JPEG_LIB_VER > 70

The invocation of this function is wrapped in an ifdef JPEGLIB >70
but the definition of the function wasn't.  This change adds the
ifdef around the function definition.