Implement missing #elif support to the preprocess_c_code() function. (#26054)

Support for `#elif` was documented in a comment to
`preprocess_c_code()`, but looks like it was forgotten to actually
implement. Add an implementation for it.

Author: juj

src/lib/libc_preprocessor.js

@@ -255,6 +255,14 @@ addToLibrary({
         var evaluated = exprTree();
         stack.push(!!evaluated * stack[stack.length-1]);
         break;
+      case 'elif':
+        var tokens = tokenize(expandMacros(expression, 0));
+        var exprTree = buildExprTree(tokens);
+        var evaluated = exprTree();
+        // If the previous #if / #elif block was executed, output NaN so that all further #elif and #else blocks will
+        // short to false.
+        stack[stack.length-1] = !!evaluated * (stack[stack.length-1] ? NaN : 1-stack[stack.length-1]);
+        break;
       case 'ifdef': stack.push(!!defs[expression] * stack[stack.length-1]); break;
       case 'ifndef': stack.push(!defs[expression] * stack[stack.length-1]); break;
       case 'else': stack[stack.length-1] = (1-stack[stack.length-1]) * stack[stack.length-2]; break;

test/test_c_preprocessor.c

@@ -191,6 +191,11 @@ EM_JS(void, test_c_preprocessor, (void), {
 
   test('#line 162 "foo.glsl"\n', '#line 162 "foo.glsl"\n'); // Test that #line directives are retained in the output
 
+  test('#define FOO 1\n#ifdef FOO\nA\n#elif defined(BAR)\nB\n#elif defined(BAZ)\nC\n#else\nD\n#endif', "A\n"); // Test #elif support, taking #ifdef path
+  test('#define BAR 1\n#ifdef FOO\nA\n#elif defined(BAR)\nB\n#elif defined(BAZ)\nC\n#else\nD\n#endif', "B\n"); // Test #elif support, taking first #elif path
+  test('#define BAZ 1\n#ifdef FOO\nA\n#elif defined(BAR)\nB\n#elif defined(BAZ)\nC\n#else\nD\n#endif', "C\n"); // Test #elif support, taking a second #elif path
+  test(               '#ifdef FOO\nA\n#elif defined(BAR)\nB\n#elif defined(BAZ)\nC\n#else\nD\n#endif', "D\n"); // Test #elif support, taking the final #else path
+
   if (numFailed) throw numFailed + ' tests failed!';
 });