Work-in-progress: add test cases for the string table APIs to the `libelftc`
test suite.

Ticket:	#361


git-svn-id: https://elftoolchain.svn.sourceforge.net/svnroot/elftoolchain/trunk@2854 95820547-d848-0410-985e-9ae8fe0fa350
Work-in-progress in `libelftc`:
 * Declare an opaque type `Elftc_String_Table`.
 * Add prototypes for the string table APIs.

Ticket:	#367

git-svn-id: https://elftoolchain.svn.sourceforge.net/svnroot/elftoolchain/trunk@2855 95820547-d848-0410-985e-9ae8fe0fa350
Code cleanups:
 * Keep a list of pre-processor symbols sorted.
 * Define convenience symbols for use as status codes.

Ticket:	#367

git-svn-id: https://elftoolchain.svn.sourceforge.net/svnroot/elftoolchain/trunk@2856 95820547-d848-0410-985e-9ae8fe0fa350