package/pkg-cmake.mk: don't unconditionally set CMAKE_CXX_COMPILER

package/pkg-cmake.mk: don't unconditionally set CMAKE_CXX_COMPILER

Don't unconditionally set CMAKE_CXX_COMPILER as it will raise a build
failure on qpid-proton because "if the toolchain specifies a value for
CMAKE_CXX_COMPILER, then CMake assumes the compiler works and goes
straight ahead trying to use it":
https://cmake.org/cmake/help/latest/module/CheckLanguage.html
https://issues.apache.org/jira/browse/PROTON-2365

Fixes:
 - http://autobuild.buildroot.org/results/05f344151100219c159ca4d466a453df96bf07fa

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