Fix build without C++

Fix the following build failure without C++:

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>

CMakeLists.txt

@@ -1,4 +1,4 @@
-PROJECT (libsolv)
+PROJECT (libsolv C)
 
 CMAKE_MINIMUM_REQUIRED (VERSION 2.8.5)
 

ext/CMakeLists.txt

@@ -103,6 +103,7 @@ IF (ENABLE_CUDFREPO)
 ENDIF (ENABLE_CUDFREPO)
 
 IF (ENABLE_HAIKU)
+    enable_language(CXX)
     SET (libsolvext_SRCS ${libsolvext_SRCS}
 	repo_haiku.cpp)
     SET (libsolvext_HEADERS ${libsolvext_HEADERS}