[C][Client] Add gcc predefined macros to reserved keywords (#19791)

docs/generators/c.md

@@ -120,6 +120,7 @@ These options may be applied as additional-properties (cli) or configOptions (pl
 <li>if</li>
 <li>inline</li>
 <li>int</li>
+<li>linux</li>
 <li>long</li>
 <li>mutable</li>
 <li>namespace</li>

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/CLibcurlClientCodegen.java

@@ -255,7 +255,11 @@ public class CLibcurlClientCodegen extends DefaultCodegen implements CodegenConf
                         // VC++ reserved keywords
                         "stdin",
                         "stdout",
-                        "stderr")
+                        "stderr",
+
+                        // gcc predefined macros
+                        "linux"
+                        )
         );
 
         instantiationTypes.clear();