[CMake] Add a workaround for #25

Because of the problem reported in #25, on Ubuntu Bionic we can't just
request LLVM 11.0.0 (or LLVM 11.0.1) with `find_package`. Instead, we
jsut add an extra check to make sure that LLVM's major version is indeed
11.

llvm-tutor will happily work with both LLVM 11.1.0 and LLVM 11.0.1.
However, on Darwin there's only LLVM 11.0.0 and on Ubuntu Bionic only
LLVM 11.1.0. So either we use:
  * find_package(LLVM 11.0.0 REQUIRED CONFIG) on Darwin
  * find_package(LLVM 11.1.0 REQUIRED CONFIG) on Ubuntu Bionic
or, we opt in for the solution implemented in this patch. Both are just
workarounds.

CMakeLists.txt

@@ -28,7 +28,13 @@ endif()
 # find_package can locate it)
 list(APPEND CMAKE_PREFIX_PATH "${LT_LLVM_INSTALL_DIR}/lib/cmake/llvm/")
 
-find_package(LLVM 11.0.0 REQUIRED CONFIG)
+find_package(LLVM REQUIRED CONFIG)
+# FIXME: This is a warkaround for #25. Remove once resolved and use
+# find_package(LLVM 11.0.0 REQUIERED CONFIG) above.
+if(NOT "11" VERSION_EQUAL "${LLVM_VERSION_MAJOR}")
+  message(FATAL_ERROR "Found LLVM ${LLVM_VERSION_MAJOR}, but need LLVM 11")
+endif()
+
 message(STATUS "Found LLVM ${LLVM_PACKAGE_VERSION}")
 message(STATUS "Using LLVMConfig.cmake in: ${LT_LLVM_INSTALL_DIR}")
 

HelloWorld/CMakeLists.txt

@@ -11,7 +11,9 @@ set(LT_LLVM_INSTALL_DIR "" CACHE PATH "LLVM installation directory")
 # find_package can locate it)
 list(APPEND CMAKE_PREFIX_PATH "${LT_LLVM_INSTALL_DIR}/lib/cmake/llvm/")
 
-find_package(LLVM 11.0.0 REQUIRED CONFIG)
+# FIXME: This is a warkaround for #25. Remove once resolved and use
+# find_package(LLVM 11.0.0 REQUIERED CONFIG) instead.
+find_package(LLVM REQUIRED CONFIG)
 
 # HelloWorld includes headers from LLVM - update the include paths accordingly
 include_directories(SYSTEM ${LLVM_INCLUDE_DIRS})