Don't override CLANG_BINARY

.travis.yml

@@ -10,7 +10,7 @@ matrix:
         apt:
           sources: ['ubuntu-toolchain-r-test', 'llvm-toolchain-trusty-4.0']
           packages: ['g++-5', 'lcov', 'llvm-4.0', 'clang-4.0', 'libclang-4.0-dev']
-      env: TOOLSET=g++-5 CPPAST_TEST_GCOV=ON LLVM_VERSION=4.0 LLVM_CONFIG_BINARY=/usr/bin/llvm-config-4.0
+      env: TOOLSET=g++-5 CPPAST_TEST_GCOV=ON LLVM_CONFIG_BINARY=/usr/bin/llvm-config-4.0
 
     - os: linux
       dist: trusty
@@ -20,12 +20,12 @@ matrix:
         apt:
           sources: ['ubuntu-toolchain-r-test', 'llvm-toolchain-trusty-4.0']
           packages: ['g++-4.9', 'g++-5', 'llvm-4.0', 'clang-4.0', 'libclang-4.0-dev']
-      env: TOOLSET=clang++-4.0 LLVM_VERSION=4.0 LLVM_CONFIG_BINARY=/usr/bin/llvm-config-4.0
+      env: TOOLSET=clang++-4.0 LLVM_CONFIG_BINARY=/usr/bin/llvm-config-4.0
 
     - os: osx
       osx_image: xcode9.2
       compiler: clang
-      env: TOOLSET=clang++ LLVM_VERSION=4.0
+      env: TOOLSET=clang++
 
 install:
   - cd ../

external/external.cmake

@@ -196,9 +196,7 @@ function(_cppast_find_libclang config_tool min_version force)
     endif()
 
     # find clang binary in llvm_binary_dir
-    if(${force})
-        unset(CLANG_BINARY CACHE)
-    endif()
+    # note: never override that binary
     if(NOT CLANG_BINARY)
         execute_process(COMMAND ${config_tool} --bindir
                         OUTPUT_VARIABLE llvm_binary_dir OUTPUT_STRIP_TRAILING_WHITESPACE)