Fix header check for glpk.h.
Squashed commit of the following:

 * Fix a bug in intersection
 * merging...
 * add delayed length lemmas
 * PreRegisterTerm is changed.
 * Bug fix for string-opt2
 * PreRegisterTerm is changed.
 * add delayed length lemmas
 * Bug fix for string-opt2
 * PreRegisterTerm is changed.
 * Bug fix for string-opt2
 * PreRegisterTerm is changed.
 * Bug fix for string-opt2
 * PreRegisterTerm is changed.
Merge pull request #41 from mdeters/tianyi-merge

Merge from Tianyi
Fixing a soundness bug in arithmetic and a roubustness problem in rings.
Alternative lazier heuristic for assertion rewriting.
Fix compiler warnings in arith and ITE utilities.
Add LETP (parallel let) to CVC4 native language.

Change CVC4 printer to use LETP by default.  CVC3 printer has no LETP.
fix #line annotations