Patch from Khoo Yit Phang.

Patch from Khoo Yit Phang.
uRemove -I../AST from CFLAGS, and change all affected #include to use relative paths.

This is in preparation to fix up automatic dependencies generation to use "gcc -MG" to handle the generated
src/AST/ASTKind.h file, which works more reliably when #include uses relative paths.

git-svn-id: https://stp-fast-prover.svn.sourceforge.net/svnroot/stp-fast-prover/trunk/stp@1124 e59a4935-1847-0410-ae03-e826735625c1