Improve external toolchain checks

This patch adds some checks on the external toolchains.

First, it checks that the C library selection is correct, by looking
if gcc is able to find the main C library file through the
-print-file-name option.

Then, it attempts to check if the Buildroot toolchain options match
the configuration of the toolchain :

 * for glibc, it checks that IPv6, RPC, locales, wide-char, large file
   support Buildroot options are enabled, since with glibc all these
   features are always available (at least this is the assumption we
   make) ;

 * for uClibc, it checks the Buildroot options with the uClibc
   configuration file in $SYSROOT_DIR/usr/include/bits/uClibc_config.h

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