update tsetcase for tokener_c

update json_visit testcase

Add test case obj token

Properly append to CMAKE_C_FLAGS string

Contrary to other CMAKE variables the CMAKE_C_FLAGS variable is the
composed string of flags for the C compiler. It is therefore not a list
to append to. Current implementation results in these incorrect CFLAGS,
e.g., "-O2 -g -fblahblah;-UNDEBUG". Extending the CFLAGS this way
results in the proper CFLAGS, e.g., "-O2 -g -fblahblah -UNDEBUG".

undefine NDEBUG for tests

Include all compiler warnings, and provide DISABLE_WERROR to make them not be errors.
 Define _REENTRANT, if setting it works.
 Set -Bsymbolic-functions, and provide DISABLE_BSYMBOLIC to turn that off.
 Implement the check for HAS_GNU_WARNING_LONG

Add vcpkg installation instructions

The second call to newlocale() with LC_TIME accidentally made things
 work because LC_TIME == LC_NUMERIC_MASK on some platforms.

Though we will no longer be comparing exactly against INT64_MAX, this is ok
because any value of that magnitude stored in a double will *also* have been
rounded up, so the comparison will work appropriately.

(closes #406)

.travis.yml:test on more recent clang and gcc versions

Maybe something weird about how gcc is configured with the test
toolchain?

(#498)

README.md:fix 2 typos