cmake: require libc++ and clang headers when building with tests or install

libc++ and clang header includes are required when running tests,
or when installing mrdocs.

Make the CMakeLists.txt enforce that requirement.

Otherwise, the tests fail in non-obvious ways.

Author: mizvekov

CMakeLists.txt

@@ -89,20 +89,23 @@ endif()
 find_package(LLVM REQUIRED CONFIG)
 find_package(Clang REQUIRED CONFIG)
 
-if (LLVM_ROOT)
-    set(LIBCXX_DIR "${LLVM_ROOT}/include/c++/v1/")
+if (MRDOCS_TEST OR MRDOCS_INSTALL)
+    set(LIBCXX_DIR "${LLVM_INCLUDE_DIR}/c++/v1/" CACHE PATH
+                   "Path to libc++ include directory")
     message(STATUS "LIBCXX_DIR: ${LIBCXX_DIR}")
     if (NOT EXISTS "${LIBCXX_DIR}")
         message(FATAL_ERROR
                 "LIBCXX_DIR (${LIBCXX_DIR}) does not exist.\n"
-                "LLVM_ROOT is set, but LIBCXX_DIR (/include/c++/v1) does not exist.\n")
+                "Please provide a LLVM with libc++ enabled\n")
     endif()
-    set(STDLIB_INCLUDE_DIR "${LLVM_ROOT}/lib/clang/${Clang_VERSION_MAJOR}/include/")
+
+    set(STDLIB_INCLUDE_DIR "${LLVM_BINARY_DIR}/lib/clang/${Clang_VERSION_MAJOR}/include/"
+                           CACHE PATH "Path to the clang headers include directory")
     message(STATUS "STDLIB_INCLUDE_DIR: ${STDLIB_INCLUDE_DIR}")
     if (NOT EXISTS "${STDLIB_INCLUDE_DIR}")
         message(FATAL_ERROR
                 "STDLIB_INCLUDE_DIR (${STDLIB_INCLUDE_DIR}) does not exist.\n"
-                "LLVM_ROOT is set, but STDLIB_INCLUDE_DIR (/lib/clang/${Clang_VERSION_MAJOR}/include) does not exist.\n")
+                "Missing clang headers\n")
     endif()
 endif()