package/libsolv: fix build without C++

Fix the following build failure without C++ raised since the addition ot
the package in commit 93ce3c1b:

CMake Error at CMakeLists.txt:1 (PROJECT):
  No CMAKE_CXX_COMPILER could be found.

  Tell CMake where to find the compiler by setting either the environment
  variable "CXX" or the CMake cache entry CMAKE_CXX_COMPILER to the full path
  to the compiler, or to the compiler name if it is in the PATH.

Fixes:
 - http://autobuild.buildroot.org/results/a7f2176d40f156c319754ef5d3b7fd0decfe754f



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