Add a BR2_NEEDS_GETTEXT option

When using an external toolchain that uses the glibc or eglibc C
libraries, compiling a separate gettext and libintl is not needed and
is even a source of confusion, causing build failures. These build
failures are due to the fact that when libintl is compiled, it
replaces the C library libintl.h by its own, which does #define
gettext libintl_gettext. Then, when packages want to use gettext,
autoconf realize that gettext is available in the C library and
therefore do not add -lintl to the LDFLAGS, causing the build failure
because the program has been compiled to use libintl_gettext but this
function is not available.

Therefore, we should only use gettext if a uClibc internal toolchain
or a uClibc external toolchain. If an external glibc toolchain is
used, gettext shouldn't be used.

In order to implement that, we introduce the BR2_NEEDS_GETTEXT option,
which is hidden to the user, and whose value is computed automatically
from the rest of the configuration.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>