Setting stm32f4 SystemCoreClock to 100MHz but can't get it (only get 57.6MHz)
It works now, here is the new code:

{
    RCC_DeInit();//a must
    do{
       RCC_HSEConfig(RCC_HSE_ON);
      }
     while (!RCC_WaitForHSEStartUp());
     RCC_PLLConfig(RCC_PLLSource_HSE, 4, 200, 4, 4);//100MHZ

    do{
       RCC_PLLCmd(ENABLE);
      }

   while(RCC_GetFlagStatus(RCC_FLAG_PLLRDY)!=SET);
   RCC_HCLKConfig(RCC_SYSCLK_Div1);
   RCC_PCLK1Config(RCC_HCLK_Div4);
   RCC_PCLK2Config(RCC_HCLK_Div2);

  RCC_SYSCLKConfig(RCC_SYSCLKSource_PLLCLK);
  SystemCoreClockUpdate();
}


With some experimentation and some more time reading the stm32f4xx_rcc.c, I discovered there was in fact a function: RCC_WaitForHSEStartUp(), which role is to wait for the HSE oscillator to start. I used it as follows:

do{
           RCC_HSEConfig(RCC_HSE_ON);
  }
         while (!RCC_WaitForHSEStartUp());


to replace in the previous version

do{
          RCC_HSEConfig(RCC_HSE_ON);;

  }

          while(RCC_GetFlagStatus(RCC_FLAG_HSERDY)!=SET);


I also "had" to start the code by initializing the clock system using the function:

 RCC_DeInit();