parse.y: fixed bison 3.0 warning

YACC  src/parse.y -> build/host/src/y.tab.c
src/parse.y:936.1-12: warning: deprecated directive, use ‘%pure-parser’
[-Wdeprecated]
%pure_parser
Merge pull request #1448 from cremno/bison-3.0-pure_parser-warning

fix bison 3.0 warning
add comments for major GC
Merge pull request #1449 from Fleurer/fix-comment-for-major-gc

add comments for major GC
remove duplicated incremental_gc_until() in generational GC; #1449
clean up the warnings when GC_PROFILE is on
def statement and define_method now return symbol a la Ruby2.1
Merge pull request #1450 from Fleurer/cleanup-warninigs

clean up the warnings when GC_PROFILE is set