package/zlib-ng: fix arm build

zlib-ng usage uses CMAKE_C_COMPILER_TARGET which is non-standard and
is not supposed to be used by the CMakeLists.txt machinery of a
particular package.

Indeed, [1] specifies that:

  Some compiler drivers are inherently cross-compilers, such as clang
  and QNX qcc. These compiler drivers support a command-line argument
  to specify the target to cross-compile for.

buildroot is not using clang nor QNX qcc.
[2] also refers to this variable only for clang [3] and QNX [4].

Therefore, zlib-ng's usage of this variable is a bit of a hack, and it's
actually why it works when passed as an argument, because they are in
fact not supposed to use this variable.

So set CMAKE_C_COMPILER_TARGET to BR2_ARCH to fix the following arm
build failure raised since bump to version 2.0.6 in commit
d2249821d3:

-- Detecting C compile features - done
-- Arch not recognized, falling back to cmake arch: 'l'
-- Basearch 'l' not recognized, defaulting to 'x86'.
-- Basearch of 'l' has been detected as: 'x86'

[...]

/home/buildroot/autobuild/instance-0/output-1/build/zlib-ng-2.1.3/arch/x86/x86_features.c:17:12: fatal error: cpuid.h: No such file or directory
   17 | #  include <cpuid.h>
      |            ^~~~~~~~~

[1] https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_COMPILER_TARGET.html
[2] https://cmake.org/cmake/help/latest/manual/cmake-toolchains.7.html
[3] https://cmake.org/cmake/help/latest/manual/cmake-toolchains.7.html#cross-compiling-using-clang
[4] https://cmake.org/cmake/help/latest/manual/cmake-toolchains.7.html#cross-compiling-for-qnx

Fixes: d2249821d3
 - http://autobuild.buildroot.org/results/1551aa69be19239a8d8e081f033e4027d679ab8f
 - http://autobuild.buildroot.org/results/075d704c0f11710353bac43478e4501addcd747d

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/zlib-ng/zlib-ng.mk

@@ -13,6 +13,7 @@ ZLIB_NG_PROVIDES = zlib
 
 # Build with zlib compatible API, gzFile support and optimizations on
 ZLIB_NG_CONF_OPTS += \
+	-DCMAKE_C_COMPILER_TARGET=$(BR2_ARCH) \
 	-DWITH_GZFILEOP=1 \
 	-DWITH_OPTIM=1 \
 	-DZLIB_COMPAT=1 \