How to define extern variable along with declaration?

All three versions of the standard — ISO/IEC 9899:1990, ISO/IEC 9899:1999 and ISO/IEC 9899:2011 — contain an example in the section with the title External object definitions (§6.7.2 of C90, and §6.9.2 of C99 and C11) which shows:

EXAMPLE 1

int i1 = 1;        // definition, external linkagestatic int i2 = 2; // definition, internal linkageextern int i3 = 3; // definition, external linkageint i4;            // tentative definition, external linkagestatic int i5;     // tentative definition, internal linkage

The example continues, but the extern int i3 = 3; line clearly shows that the standard indicates that it should be allowed. Note, however, that examples in the standard are technically not 'normative' (see the foreword in the standard); they are not a definitive statement of what is and is not allowed.

That said, most people most of the time do not use extern and an initializer.

This code is perfectly valid.

But any compiler is free to issue additional (informative or not) diagnostics:

(C99, 5.1.1.3p1 fn 8) "Of course, an implementation is free to produce any number of diagnostics as long as a valid program is still correctly translated."

What a compiler cannot do is not emitting a diagnostic when there is a constraint or syntax violation.

EDIT:

As devnull put in the OP question comments, Joseph Myers from gcc team explains in a bug report questioning this diagnostic:

"This is a coding style warning - the code is valid, but extremely unidiomatic for C since "extern" is generally expected to mean that the declaration is not providing a definition of the object."