Both the C89 (section 3.5.2.2) and C99 (section 6.7.2.2) standards define enums the same way:
6.7.2.2 Enumeration specifiers (Paragraph 3), http://www.open-std.org/JTC1/SC22/WG14/www/docs/n1256.pdf
3.5.2.2 Enumeration specifiers (Paragraph 3), http://flash-gordon.me.uk/ansi.c.txt
Both read:
[...] An enumerator with = defines its enumeration constant as the value of the constant expression. If the first enumerator has no =,the value of its enumeration constant is 0. Each subsequent enumerator with no = defines its enumeration constant as the value of the constant expression obtained by adding 1 to the value of the previous enumeration constant. [...]
Therefore, in your syntax, any standard-compliant compiler will run your code correctly.