build: fix codegen errors when using static linkage

Commit 1670a89 upgraded cmake from 3.2 to 3.6, and in the process, broke all
statically linked codegen-dependent Kudu executables. When run, they
produced errors like the following:

  LLVM ERROR: Program used external function '_ZN4kudu9ArenaBaseILb0EE21AllocateBytesFallbackEmm' which could not be resolved!

The release notes revealed the culprit: CMP0065[1]. This policy change
removed -rdynamic from every Kudu executable's link line, which had the side
effect of breaking codegen. I don't know why we need -rdynamic, but I
verified that restoring it manually (and using this patch to restore it) fix
the tests. I went digging for better solutions and saw that maybe we should
call ExecutionEngine::addGlobalMapping() on our created functions[2][3], but
after looking at the hairy mess that is JITWrapper, I decided to punt.

This wasn't caught in precommit tests because gerrit uses distributed
testing, which means release builds are dynamically linked to reduce
executable disk space consumption.

1. https://cmake.org/cmake/help/v3.4/policy/CMP0065.html
2. http://stackoverflow.com/questions/4425797/linking-llvm-jit-code-to-external-c-functions
3. https://llvm.org/bugs/show_bug.cgi?id=20656

Change-Id: Ie803b93cfd9b5bed56ece5f24b769850b7b55b74
Reviewed-on: http://gerrit.cloudera.org:8080/4540
Tested-by: Kudu Jenkins
Reviewed-by: Alexey Serbin <[email protected]>

CMakeLists.txt

@@ -87,6 +87,14 @@ set(BUILD_SUPPORT_DIR ${CMAKE_CURRENT_SOURCE_DIR}/build-support)
 # Must be declared in the top-level CMakeLists.txt.
 set(CMAKE_SKIP_INSTALL_ALL_DEPENDENCY true)
 
+# Codegen-dependent executables need to be linked with -rdynamic; otherwise LLVM
+# can't find dependent Kudu symbols at runtime.
+#
+# Setting the ENABLE_EXPORTS target property for each codegen-dependent
+# executable is more precise, but that's a rather long target list, so we'll
+# just do it once here for all Kudu executables.
+set(CMAKE_ENABLE_EXPORTS true)
+
 # Make sure thirdparty stuff is up-to-date.
 if ("$ENV{NO_REBUILD_THIRDPARTY}" STREQUAL "")
   execute_process(