Lately I've settled on eclipse CDT as my primary IDE for C++ development. It has it's own built-in incremental C++ compiler which doesn't always exactly agree with GCC.

I came across this snippet while brushing off some old code:

GCC compiles this just fine. However, the eclipse compiler signals a "syntax error" on the const array initializer (marked with a comment). Looking at this code, I don't know what I was thinking. Why is this way of initializing structs even compiling at all in GCC? Is this a non-standard extension of the language?

It's probably a non-standard extension like you said. The standard way to do the same in C99 looks like this:

struct Blah {
 int a, b, c;
};

const struct Blah blah =
{
  .a = 10,
  .b = 20,
  .c = 30,
};

I don't think that works in C++ though.
If Neil is correct about Eclipse using GCC, you may just need to enable GCC extensions in your IDE and it will compile again.

I didn't know that.

I just gave up on CTD after I realised it wanted to force the Java folder/file hierarchy and project management system on me and it simply didn't fit with C++ so I moved to NetBeans.

Why create extensions, just stick with the fecking rules, they're there for a reason.

Because why not?
Nothing wrong with giving you a few more useful options. It's up to you if you want to use them or stick to the standard.

Someone felt the need for something, compiler authors implement it as an extension and see if it works. If enough people (or the right people) think the feature is worthwhile then it gets codified into a standard, using the lessons learnt from the extension.

I think it was I wanted to include the source code from other projects in the build but keep them in a different folder. CTD either wanted me to simply link to the library files or have the code as a completely new eclipse project, neither of which is what I wanted to do.