package/pugixml: add support for a limited set of configuration options

Add config options for a few pugixml configurables.

- Xpath support is enabled by default but has a size impact. Disabling it
  reduces the size significantly (it almost halves). Output of 'size' on the
  library compiled for x86:
  - Xpath support enabled
 160374	   1244	     28	 161646	  2776e	output/target/usr/lib/libpugixml.so.1.7
  - Xpath support disabled
  92754	    880	      8	  93642	  16dca	usr/lib/libpugixml.so.1.7

- Compact and header-only modes are not strictly needed for our use case, but we
  did the work anyway and may be useful for someone else.

Signed-off-by: Wouter Vermeiren <wouter.vermeiren@nokia.com>
[ThomasDS:
- align with Buildroot coding style
- retain only feature options: xpath, compact mode, header-only]
Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Tested-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>