Updates to configure.ac and Makefile

[jpco]

I may have touched more than I should have with this change.

configure.ac:

• Add --enable-strict option for ./configure; this turns on the "strict" CFLAGS -ansi -pedantic -D_POSIX_C_SOURCE=200112L -DGCDEBUG=1 -DREF_ASSERTIONS=1
• Remove AC_HEADER_STDC, which only has the effect of generating a warning from autoconf.

Makefile.in:

• Remove -Wdeclaration-after-statement, as it's now included in the "strict" warnings.
• Remove old, obsolete comment about bison generating bad parsers and bring back explicit YACC variable.
• Move $(ADDCFLAGS) and friends to the end of their respective CFLAGS and friends lines, as compiler commands largely seem to follow last-takes-precedence.
• Have make test run ./es with -p now, for better test consistency.
• Don't delete ./testrun with make clean.
• Convert ${FOO}s to $(FOO)s for the sake of consistency.
• Delete ragged end-of-line spaces.

CircleCI config:

• Run CircleCI tests with -p for symmetry with make test.
• Use --enable-strict instead of manually feeding ADDCFLAGS to make.