Clarify build directions; make unit tests buildable with system LLVM (#93)

Author: hikari-no-yume

README.md

@@ -38,7 +38,7 @@ Note: to convert C to LLVM IR to run the tests, you will also need a C compiler
 The first step is to compile LLVM on your machine
 (this assumes an in-tree build, but out-of-tree will also work):
 
-     cd $HOME
+
      git clone https://github.com/llvm/llvm-project.git
      cd llvm-project
      git checkout release/8.x
@@ -53,14 +53,23 @@ To run tests, you need to build `lli`.
 Step 2: Compiling LLVM-CBE
 ==========================
 
-Next, download and compile llvm-cbe from the same folder if you built LLVM yourself, or where you'd like in the other case:
+Now you can download and compile llvm-cbe.
+
+If you built LLVM yourself, put it in the same folder you built LLVM in:
 
-    cd $HOME/llvm-project/llvm/projects # Or any place, if you use the LLVM package of your distribution.
+    cd $HOME/llvm-project/llvm/projects
     git clone https://github.com/JuliaComputing/llvm-cbe
     cd ../build
     cmake ..
     make llvm-cbe
 
+If you used your distribution's package, put it wherever you feel like:
+
+    git clone https://github.com/JuliaComputing/llvm-cbe
+    cd llvm-cbe && mkdir build && cd build
+    cmake ..
+    make llvm-cbe
+
 Step 3: Usage Examples
 ======================
 
@@ -98,6 +107,10 @@ Unit tests:
     $ make CBEUnitTests && projects/llvm-cbe/unittests/CWriterTest
 ```
 
+Note that you need to have passed `-DLLVM_INCLUDE_TESTS=1` to cmake if you used
+your distribution's LLVM package. You also will need to install gtest (on Debian
+derivatives: `apt install libgtest-dev`).
+
 Other tests:
 
 First, compile llvm-cbe, and install pytest. Then:

unittests/CMakeLists.txt

@@ -1,19 +1,29 @@
 add_custom_target(CBEUnitTests)
 set_target_properties(CBEUnitTests PROPERTIES FOLDER "Tests")
 
-set(LLVM_LINK_COMPONENTS
-  Analysis
-  CBackendCodeGen
-  CBackendInfo
-  CodeGen
-  Core
-  MC
-  ScalarOpts
-  Support
-  Target
-  TransformUtils
-  )
+if (NOT USE_SYSTEM_LLVM)
+  set(LLVM_LINK_COMPONENTS
+    Analysis
+    CBackendCodeGen
+    CBackendInfo
+    CodeGen
+    Core
+    MC
+    ScalarOpts
+    Support
+    Target
+    TransformUtils
+    )
+endif()
 
 add_unittest(CBEUnitTests CWriterTest
   TypeNameTest.cpp
   )
+
+if(USE_SYSTEM_LLVM)
+  llvm_map_components_to_libnames(llvm_libs
+    CBackendCodeGen
+    CBackendInfo
+    )
+  target_link_libraries(CWriterTest PRIVATE LLVM ${llvm_libs})
+endif()