[cmake] Add test for inline variables

Inline variables are a C++17 feature that gives variables the same
type of linkage as inline functions, which avoids object file bloat
and linker problems.

For us, this is mostly interesting in the context of constexpr
variables like the partition sets provided in dune-grid: Right now,
a separate copy of those objects is emitted by each translation unit,
and those copies do not get merged by the linker. This has created
problems in the past and forced us to place the variables in an
anonymous namespace, which creates unique symbol names for each
translation unit.

With inline variables, the problem will eventually go away.

cmake/modules/CheckCXXFeatures.cmake

@@ -481,3 +481,15 @@ check_cxx_source_compiles("
   }
 " DUNE_SUPPORTS_CXX_THROW_IN_CONSTEXPR
   )
+
+
+# Check whether the compiler supports inline variables
+check_cxx_source_compiles("
+  inline int foo = 42;
+
+  int main()
+  {
+    return not (foo == 42);
+  }
+" DUNE_HAVE_CXX_INLINE_VARIABLES
+  )

config.h.cmake

@@ -35,6 +35,9 @@
 /* does the compiler support conditionally throwing exceptions in constexpr context? */
 #cmakedefine DUNE_SUPPORTS_CXX_THROW_IN_CONSTEXPR 1
 
+/* does the compiler support inline variables? */
+#cmakedefine DUNE_HAVE_CXX_INLINE_VARIABLES 1
+
 /* Define if you have a BLAS library. */
 #cmakedefine HAVE_BLAS 1