add explicit CXX language setting in CMake

The default is C CXX, which pulls in an unnecessary dependency on a C
compiler. While this is mostly irrelevant, this breaks the build in
Spack >= 1.0 or forces us to add an explicit dependency on "c" there.

Author: tukss

CMakeLists.txt

@@ -6,7 +6,8 @@ project(
   ${CODIPACK_NAME}
   VERSION ${CODIPACK_VERSION}
   DESCRIPTION "CoDiPack - Code Differentiation Package"
-  HOMEPAGE_URL "http://scicomp.rptu.de/software/codi")
+  HOMEPAGE_URL "http://scicomp.rptu.de/software/codi"
+  LANGUAGES CXX)
 
 add_library(${PROJECT_NAME} INTERFACE)