Fixed following MSVC warning/errors when compiled as C source:

[r-lyeh]

• error C2057: expected constant expression

• error C2059: syntax error: '{}'

• error C2065: 'sample11': undeclared identifier

• error C2133: 'distance_rows': unknown size

• error C2466: cannot allocate an array of constant size 0

• warning C4098: 'j40__add_thresholds': 'void' function returning a value

• warning C4098: 'j40__forward_dct_unscaled': 'void' function returning a value

• warning C4098: 'j40__inverse_dct': 'void' function returning a value

• warning C4141: 'inline': used more than once

• [x] I agree to publish my contributions to the public domain and in any public-domain equivalent license described in LICENSE.txt.

[r-lyeh]

Feel free to discard completely and/or cherry-pick fixes from this. Amazing project btw!

[lifthrasiir]

Thank you for PR. It took a while because I had to make the build system fully support Windows (please tell me whether make works out of the box or not!). I incorporated your changes as 501f733e231e0248e72e1068ec08fc11f5045b44 except for _lzcnt_u32/64, which is a machine intrinsic and should not be used in a portable code and I rewrote that part in 095f4a3afc36de39db7dfdb751c6a2e086be3388.