CMakeLists: Fixup boost

* boost is part of the public interface.
* Consider boost a system library so warnings from boost do not cause a build failure.
* If the parent project defines boost, use that.

.travis.yml

@@ -17,6 +17,6 @@ matrix:
       script: ./.travis/build-x86_64-linux.sh
     - os: osx
       sudo: false
-      osx_image: xcode7.3
+      osx_image: xcode9.2
       install: ./.travis/deps-x86_64-macos.sh
       script: ./.travis/build-x86_64-macos.sh

CMakeLists.txt

@@ -104,15 +104,14 @@ endif()
 message(STATUS "Target architecture: ${ARCHITECTURE}")
 
 # Include Boost
-if(DYNARMIC_USE_SYSTEM_BOOST)
-    find_package(Boost 1.57.0 REQUIRED)
-else()
-    if(NOT Boost_INCLUDE_DIRS)
-        message(FATAL_ERROR "Please provide a path to a boost installation using Boost_INCLUDE_DIRS")
+if (NOT TARGET boost)
+    if (NOT Boost_INCLUDE_DIRS)
+        find_package(Boost 1.57.0 REQUIRED)
     endif()
+    add_library(boost INTERFACE IMPORTED)
+    set_property(TARGET boost PROPERTY INTERFACE_INCLUDE_DIRECTORIES ${Boost_INCLUDE_DIRS})
+    set_property(TARGET boost PROPERTY INTERFACE_SYSTEM_INCLUDE_DIRECTORIES ${Boost_INCLUDE_DIRS})
 endif()
-add_library(boost INTERFACE IMPORTED)
-set_property(TARGET boost PROPERTY INTERFACE_INCLUDE_DIRECTORIES ${Boost_INCLUDE_DIRS})
 
 # Enable unit-testing.
 enable_testing(true)

src/CMakeLists.txt

@@ -112,8 +112,9 @@ target_include_directories(dynarmic
                            PRIVATE .)
 target_compile_options(dynarmic PRIVATE ${DYNARMIC_CXX_FLAGS})
 target_link_libraries(dynarmic
-    PRIVATE
+    PUBLIC
         boost
+    PRIVATE
         fmt::fmt
         xbyak
         $<$<BOOL:DYNARMIC_USE_LLVM>:${llvm_libs}>