lib: simplify main build file

lib: simplify main build file

Two simplifications can be made to the build file which reduce indentation
levels and make it easier to read:

1. When meson build support was first added, the compat library existed in
DPDK as a single header file. Since that header has been merged into EAL,
we no longer need to support header-only libraries, so can shorten the
code.

2. From meson 0.49 onwards we have the "continue" keyword available to
break out of one loop iteration and begin the next. This allows us to
remove blocks in the build configuration file which were conditional on the
"build" variable being true. Instead we can use "continue" to abort
processing at the point where the "build" value becomes false.

Since this patch changes the indentation level of large parts of the
meson.build file, we use the opportunity to adjust the whitespace used to
the meson-standard 4-spec indentation level.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>