Update .travis.yml

Use `LLVM_DIR` rather than `CMAKE_PREFIX_PATH` to locate LLVM.  This is more
modern, and avoids the problem of searching for multiple versions as we have
provided an absolute path to the location of the CMake module for LLVM.

Simplify the CMake by unversioning the check for `find_package` and manually
ensuring that we have a new enough LLVM.

.travis.yml

@@ -40,7 +40,7 @@ jobs:
         - mkdir build && cd build
         - CC=gcc-4.8 CXX=g++-4.8 cmake -G Ninja
           -DCMAKE_BUILD_TYPE=Debug -DGLOW_WITH_OPENCL=OFF -DGLOW_WITH_CPU=ON
-          -DCMAKE_PREFIX_PATH=/usr/lib/llvm-7/include/
+          -DLLVM_DIR=/usr/lib/llvm-7/cmake
           -DCMAKE_CXX_FLAGS=-Werror
           -DGLOW_USE_COVERAGE=ON
           ../

CMakeLists.txt

@@ -105,20 +105,9 @@ endif ()
 # Top level setup for external backends
 ExternalBackendsInit()
 
-find_package(LLVM 8 CONFIG)
-if (NOT LLVM_FOUND)
-  # Fallback to LLVM 7
-  find_package(LLVM 7 CONFIG)
-
-  if (NOT LLVM_FOUND)
-    # Fallback to whatever is available.
-    find_package(LLVM CONFIG)
-
-    if (NOT LLVM_FOUND OR LLVM_PACKAGE_VERSION VERSION_LESS "7.0")
-      message(FATAL_ERROR "LLVM minimum required version is 7. LLVM package version found: ${LLVM_PACKAGE_VERSION}.")
-    endif()
-
-  endif()
+find_package(LLVM CONFIG)
+if(NOT LLVM_FOUND OR LLVM_VERSION VERSION_LESS 7.0)
+  message(SEND_ERROR "LLVM >= 7.0 is required to build Glow")
 endif()
 
 message(STATUS "Found LLVM ${LLVM_PACKAGE_VERSION}")