Code:

#include "usbdrv.h"

void f( void )
{
    int schar = 123; // error: two or more data types in declaration specifiers
}

Code:

typedef signed char schar;

void f( void )
{
    int schar = 123; // no error
}

Code:

// other.h
#define schar signed char

// usbdrv.h
#ifndef schar // no problem, already defined so this is skipped
#define schar signed char
#endif

Code:

#ifndef schar
#define schar schar
typedef signed char schar;
#endif

void f( void )
{
    int schar = 123; // no error
}