sscanf doesn't handle hexadecimal floating-point input

[Keith Thompson]

The *scanf() functions don't handle hexadecimal floating-point input
(for example "0x1p+0" representing 1.0).

A test program:

#include <stdio.h>
int main(void) {
    int ok = 1;
    float x = -1.0;
    int result = sscanf("0x1p+0", "%f", &x);
    const float expected_x = 1.0;
    const int expected_result = 1;
    printf("sscanf returned %d", result);
    if (result != expected_result) {
        ok = 0;
        printf(" (expected %d)", expected_result);
    }
    printf(", x = %g", x);
    if (x != expected_x) {
        ok = 0;
        printf(" (expected %g)", expected_x);
    }
    puts(ok ? ", PASSED" : ", FAILED");
}

On Cygwin, the output (compiled with gcc or clang) is:

sscanf returned 1, x = 0 (expected 1), FAILED

On Ubuntu, the output is:

sscanf returned 1, x = 1, PASSED

Looking through the newlib sources (git://sourceware.org/git/newlib-cygwin.git),
this might be related to the _WANT_IO_C99_FORMATS macro, but I haven't
looked into
the details.

The test case passes on Cygwin when compiled with i686-w64-mingw32-gcc
or x86_64-w64-mingw32-gcc.