Generate compile_commands.json

This file is used with tooling like IDEs to get autocompletion and other
features that need to be aware of compile flags used on each source
file. Seems to affect build performance minimally, and only takes a few
MB of disk space when populated. I think it makes sense to turn it on
globally so that it's there when tools/IDEs need it.

.gitignore

@@ -94,3 +94,8 @@ deps/build
 doc/
 
 /dummy.cpp
+
+# compile_commands.json - http://clang.llvm.org/docs/JSONCompilationDatabase.html
+# generated by CMake + clang - Symlink debug/compile_commands.json (or release)
+# to the root of the project for most tooling to work. This ignores the symlink.
+/compile_commands.json

CMakeLists.txt

@@ -16,6 +16,10 @@ cmake_policy(SET CMP0046 OLD)
 cmake_policy(SET CMP0042 NEW)
 endif()
 
+# Generate a compilation database for use with automated tools like IDE/editor
+# plugins. See http://clang.llvm.org/docs/JSONCompilationDatabase.html
+set( CMAKE_EXPORT_COMPILE_COMMANDS ON )
+
 # Determine where additional Turi specific cmake modules are
 # defined
 set(DEPS_CMAKE ${CMAKE_SOURCE_DIR}/deps/cmake)