configure: rationalise error messages with meaningful texts

Currently, error messages just state the obvious: a required
component is required. While in some cases, there is not much
else to say, in some other cases we could tell much more.

For example, a missing libstdc++ means no static toolchain.

Also, format error and warning messages in a similar fashion,
with a {error,warning} intro, followed by the message specified
by the caller.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

mainmenu "The crosstool-NG configuration menu"

source "config/configure.in"

source "config/backend.in"

source "config/global.in"

source "config/target.in"

source "config/toolchain.in"

source "config/kernel.in"

source "config/binutils.in"

source "config/cc.in"

source "config/libc.in"

source "config/debug.in"

source "config/companion_libs.in"

source "config/companion_tools.in"

source "config/test_suite.in"