Remember compiler options during attach.

Note:
- Move compiler options into `mjsCompiler` struct.
- The compiler options from the attached model are not written by mj_SaveXML.
PiperOrigin-RevId: 689391440
Change-Id: I1d63c146a32f87c737b7a55b64a54b0ffe3aecc9

src/user/user_flexcomp.h

@@ -42,7 +42,7 @@ typedef enum _mjtFcompType {
 class mjCFlexcomp {
  public:
   mjCFlexcomp(void);
-  bool Make(mjSpec* spec, mjsBody* body, char* error, int error_sz);
+  bool Make(mjsBody* body, char* error, int error_sz);
 
   bool MakeGrid(char* error, int error_sz);
   bool MakeBox(char* error, int error_sz);