package/Makefile.in: add detection for the lack of C library

We recently had several cases of architecture configurations for which
no C library was available, leading to a build failure during the gcc
build. In order to more easily detect those bogus configurations,
let's bail out very early by detecting the lack of C library
selection.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[yann.morin.1998@free.fr:
  - move as final else clause in existing conditional block
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/Makefile.in

@@ -51,8 +51,12 @@ ifeq ($(BR2_TOOLCHAIN_USES_UCLIBC),y)
 LIBC = uclibc
 else ifeq ($(BR2_TOOLCHAIN_USES_MUSL),y)
 LIBC = musl
-else
+else ifeq ($(BR2_TOOLCHAIN_USES_GLIBC),y)
 LIBC = gnu
+else
+# This happens if there is a bug in Buildroot that allows an
+# architecture configuration that isn't supported by any library.
+$(error No C library enabled, this is not possible.)
 endif
 
 # The ABI suffix is a bit special on ARM, as it needs to be