build-sys: make the crosscompile path for scanf_cv_alloc_modifier clever

build-sys: make the crosscompile path for scanf_cv_alloc_modifier clever

My earlier assumption was that scanf_cv_alloc_modifier is unused. This
is not true as pointed out by Karel since it used befoe checking for
libmount. Argh. So instead saying no I add a test to check for glibc
>= 2.7 which provides %m. As of uClibc v0.9.32-rc3 the situation looks
the following:
| git grep _M_SPEC__
|libc/stdio/_vfprintf.c:/* #define __UCLIBC_HAS_PRINTF_M_SPEC__ */
|libc/stdio/_vfprintf.c:#ifdef __UCLIBC_HAS_PRINTF_M_SPEC__
|libc/stdio/_vfprintf.c:# ifdef __UCLIBC_HAS_PRINTF_M_SPEC__
|libc/stdio/_vfprintf.c:# endif /* __UCLIBC_HAS_PRINTF_M_SPEC__ */
|libc/stdio/_vfprintf.c:#ifdef __UCLIBC_HAS_PRINTF_M_SPEC__
|libc/stdio/_vfprintf.c:#ifdef __UCLIBC_HAS_PRINTF_M_SPEC__
|libc/stdio/_vfprintf.c:#ifdef __UCLIBC_HAS_PRINTF_M_SPEC__
|libc/stdio/_vfprintf.c:#ifdef __UCLIBC_HAS_PRINTF_M_SPEC__
|libc/stdio/old_vfprintf.c:#ifdef __UCLIBC_HAS_PRINTF_M_SPEC__
|libc/stdio/perror.c:#ifdef __UCLIBC_HAS_PRINTF_M_SPEC__

And other libc user have to bring their own stuff.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>