Compat fixes

[sherpya]

there is no need to call CryptAcquireContextW because two string parameters are NULL
CryptAcquireContextW does not work on Win98, and CRYPT_SILENT flags is not supported on WInNT4 and Win98x, any info about WIn2k?

I'm returning the fallback function, if you don't like it it can be reverted, but anyway is a bad pratice to call exit() inside a library

[sherpya]

do you prefer also against master?

[coveralls]

Coverage remained the same at 86.758% when pulling 5bb5842 on clamwin:compat-fixes into 4467e94 on json-c:json-c-0.14.

[hawicz]

Yes, if you could re-spin this against master that would be preferred, since this isn't a 0.14-specific change.
Also, I'm expecting to release a 0.15 version before a 0.14.1 (if at all), so it might not be worth pulling over to the 0.14 branch at all.