json-c: Update to 0.12

Please take a look at src/json-c-3-remove-w32-crypt-function.patch
before accepting the pull request, this may have security implications
They are available in MXE.

i686-pc-mingw32.shared:

```
timothy_gu@ubuntu-lenovo:~/mxe/usr/i686-pc-mingw32.shared/lib$ nm libadvapi32.a  | grep CryptAcquireContextW
00000000 T _CryptAcquireContextW@20
00000000 I __imp__CryptAcquireContextW@20
timothy_gu@ubuntu-lenovo:~/mxe/usr/i686-pc-mingw32.shared/lib$ nm libadvapi32.a  | grep CryptGenRandom
00000000 T _CryptGenRandom@12
00000000 I __imp__CryptGenRandom@12
timothy_gu@ubuntu-lenovo:~/mxe/usr/i686-pc-mingw32.shared/lib$ nm libadvapi32.a  | grep CryptReleaseContext 
00000000 T _CryptReleaseContext@8
00000000 I __imp__CryptReleaseContext@8
```

x86_64-mingw32-w64.static:

```
timothy_gu@ubuntu-lenovo:~/mxe/usr/x86_64-w64-mingw32.static/lib$ nm libadvapi32.a  | grep CryptAcquireContextW
0000000000000000 T CryptAcquireContextW
0000000000000000 I __imp_CryptAcquireContextW
timothy_gu@ubuntu-lenovo:~/mxe/usr/x86_64-w64-mingw32.static/lib$ nm libadvapi32.a  | grep CryptGenRandom
0000000000000000 T CryptGenRandom
0000000000000000 I __imp_CryptGenRandom
timothy_gu@ubuntu-lenovo:~/mxe/usr/x86_64-w64-mingw32.static/lib$ nm libadvapi32.a  | grep CryptReleaseContext 
0000000000000000 T CryptReleaseContext
0000000000000000 I __imp_CryptReleaseContext
```

However, because json-c uses a nonstandard MSVC-only `#pragma`, fixing this for MXE will be harder. Here is an IMO cleaner and more upstream-applicable approach, as easy as 1-2-3™ ;):

1. Guard the pragma with `#ifdef _MSC_VER`
2. Add a `@ADVAPI32_LIB@` placeholder in pkgconfig file and Makefile.am.
3. Add a check in configure for `CryptAcquireContextW`, with `-ladvapi32` temporarily added. If found, add `-ladvapi32` to `ADVAPI32_LIB` variable, and `AC_SUBST(ADVAPI32_LIB)`.