In C99, one can define pragma by using _Pragma operator.
It helps to unify several pragmas with the same function.
e.g.

#if defined(VendorA)
# define pragma_fastcall _Pragma("call_func_fast")
#elif defined(VendorB)
# define pragma_fastcall _Pragma("fastcall")
#elif defined(VendorC)
# define pragma_fastcall _Pragma("__fastcall")
#endif

pragma_fastcall void foo() { ... }
pragma_fastcall void bar() { ... }
pragma_fastcall void baz() { ... }


I suggest user-defined pragmas, e.g.:

pragma User_foo(param) {
    version(VendorA) {
        pragma(VendorA_foo, param);
    } else version(VendorB) {
        pragma(VendorB_bar, param, 0);
    } else version(VendorC) {
        pragma(VendorC_foo, 0, param);
    } else {
        pragma(error, "User_foo: not supported.");  // I also suggest error
pragmas.
    }
}

pragma(User_foo, 4) struct A { int a, b, c; }
pragma(User_foo, 8) struct B { int a, b, c; }
pragma(User_foo, 16) struct C { int a, b, c; }