add piston nix pkg for gcc-c

runtimes/gcc-c.nix

@@ -0,0 +1,41 @@
+{pkgs, piston, ...}:
+let
+    pkg = pkgs.gcc;
+in piston.mkRuntime {
+    language = "c";
+    version = pkg.version;
+    runtime = "gcc";
+
+    aliases = [
+        "gcc"
+    ];
+
+    compile = ''
+        rename 's/$/\.c/' "$@" # Add .c extension
+        ${pkg}/bin/gcc -std=c11 *.c -lm
+    '';
+
+    run = ''
+        shift
+        ./a.out "$@"
+    '';
+
+    tests = [
+        (piston.mkTest {
+            files = {
+                "test.c" = ''
+                    #include <stdio.h>
+
+                    int main(void) {
+                        printf("OK");
+                        return 0;
+                    }
+                '';
+            };
+            args = [];
+            stdin = "";
+            packages = [];
+            main = "test.c";
+        })
+    ];
+}