Eliminate a few more easy warnings

Testing with 32-bit clang 3.3, with additional compiler flags
 -Wsign-compare -Wundef
this patch eliminates the following warnings:

config.h:100:6: warning: 'UINT64_T_AND_ULONG_SAME' is not defined, evaluates to 0 [-Wundef]
vcd_priv2.cc:233:12: warning: duplicate 'extern' declaration specifier [-Wduplicate-decl-specifier]
parse.cc:6496:9: warning: comparison of 0 <= unsigned expression is always true [-Wtautological-compare]
parse.cc:6499:13: warning: comparison of 0 <= unsigned expression is always true [-Wtautological-compare]
parse.cc:6502:9: warning: comparison of 0 <= unsigned expression is always true [-Wtautological-compare]
parse.cc:6510:53: warning: comparison of integers of different signs: 'const unsigned int' and 'int' [-Wsign-compare]

Changing the vlltype elements from unsigned to int reconciles their
type with the native bison YYLTYPE structure.