Do not disable GNU extensions when building grammars

Commit ca65d31 ("always build
grammars with c++14 and c11 (helix-editor#6792)") turned on semi-strict
standard mode for C and C++ by accident.  Some grammars use
non-standard functions such as isascii (from <ctype.h>) and
fail to build as a result, particularly with future compilers
which do not support implicit function declarations.

(Ideally, the build environment would only raise the standard
versions relative to the toolchain default, not lower them, but
this change does not implement that.)

helix-loader/src/grammar.rs

@@ -509,7 +509,7 @@ fn build_tree_sitter_library(
 
         if let Some(scanner_path) = scanner_path.as_ref() {
             if scanner_path.extension() == Some("c".as_ref()) {
-                command.arg("-xc").arg("-std=c11").arg(scanner_path);
+                command.arg("-xc").arg("-std=gnu11").arg(scanner_path);
             } else {
                 let mut cpp_command = Command::new(compiler.path());
                 cpp_command.current_dir(src_path);
@@ -526,7 +526,7 @@ fn build_tree_sitter_library(
                     .arg(header_path)
                     .arg("-o")
                     .arg(&object_file)
-                    .arg("-std=c++14")
+                    .arg("-std=gnu++14")
                     .arg("-c")
                     .arg(scanner_path);
                 let output = cpp_command
@@ -544,7 +544,7 @@ fn build_tree_sitter_library(
                 _path_guard = TempPath::from_path(object_file);
             }
         }
-        command.arg("-xc").arg("-std=c11").arg(parser_path);
+        command.arg("-xc").arg("-std=gnu11").arg(parser_path);
         if cfg!(all(
             unix,
             not(any(target_os = "macos", target_os = "illumos"))