package/Makefile.in: only error out when no C library is configured when building

Commit fda53f0791f5657d41735a40a29df3c8d0e8ae79 ("package/Makefile.in:
add detection for the lack of C library") added an $(error ...)
message when no C library is available for the currently selected
architecture.

However, this error message pops up not just when building, so for
example, the command:

  make BR2_HAVE_DOT_CONFIG=y VARS=%_LICENSE printvars

no longer works (this command is used by the pkg-stats script).

We restore a functional behavior by doing the check only when
BR_BUILDING=y.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/Makefile.in

@@ -53,7 +53,7 @@ else ifeq ($(BR2_TOOLCHAIN_USES_MUSL),y)
 LIBC = musl
 else ifeq ($(BR2_TOOLCHAIN_USES_GLIBC),y)
 LIBC = gnu
-else
+else ifeq ($(BR_BUILDING),y)
 # 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.)