add clang to tools if necessary

See merge request hammer/hammer!50

Ticket 95:  Fix backends with params that currently ignore requested params to use them.

Closes #95

See merge request !49

Lets one, e.g., add -fPIC if compiling with GCC but linking with LLD.

clang appears to require -fPIC for shared objects on most platforms. the
'clang' tool adds the flag when appropriate. however, scons never selects the
clang tool automatically!

so we add the tool ourselves if CC=clang is set or generally on Darwin.

ideally, scons would know and handle all this, but at the time of writing
(scons v4.3.0), it does not.

WIP: arena allocator improvements

See merge request hammer/hammer!30

default of k=1 is used.

Add ABNF-parser-derived bug60 test case, and improve some of the test macros

See merge request !45

Ticket 60 bugfix

See merge request hammer/hammer!43

that we also check to confirm both engines have consume the same number
of tokens (and thus have actually both arrived at the same state for the
same partial input stream)

Fix uninitialized symbol warning in cfgrammar.c

See merge request !42

Fix feature macros

See merge request !41

Restore -D_POSIX_C_SOURCE=200809L in build flags on Linux; we'll figure out report of Mac OS breakage later

Using _POSIX_C_SOURCE in the middle of a header is incorrect and may work unreliably because of multiple-include guards

add regression test for issue 87 - same as issue 91

Closes #87

See merge request !37

Resolve "empty follow set assertion triggered with LALR"

Closes #92

See merge request !35

add TT_DOUBLE and TT_FLOAT as user-accessible token types

See merge request !32

Resolve "_POSIX_C_SOURCE breaks build on MacOS X"

Closes #93

See merge request !38

Resolve assertion failure with rules that can never match

Closes #83

See merge request hammer/hammer!39

fixes issue #83

#83

doh! fixes issue 92 again.

tentative commit. this should solve issue 92, but isn't working, yet.

the idea is to taint any memoized follow set that hit a recursive loop,
directly or indirectly. the recursive calls (h_follow_rec) are allowed to use
these intermediates, but propagate the taint. the top-level routine (h_follow)
places the original taint on its result and may remove it at the end. it will
not return memoized results that are tainted (but compute them afresh).