Try to fix libunwind build issue

My guess is that there's a build dependency issue, where
we try building files that include libunwind.h before we
run the rules that generate it.  Not sure why we weren't running
into that before.  But the normal c++ files in src/ correctly
depend on libunwind, but the ones in src/runtime/inline don't
seem to.

For good measure, don't include libunwind.h in our header files,
and just use forward-declaring.

src/codegen/unwinding.cpp

@@ -46,6 +46,8 @@
 
 #define UNW_LOCAL_ONLY
 #include <libunwind.h>
+#undef UNW_LOCAL_ONLY
+
 namespace {
 int _dummy_ = unw_set_caching_policy(unw_local_addr_space, UNW_CACHE_PER_THREAD);
 }

src/codegen/unwinding.h

@@ -19,9 +19,9 @@
 
 #include "codegen/codegen.h"
 
-#define UNW_LOCAL_ONLY
-#include <libunwind.h>
-#undef UNW_LOCAL_ONLY
+// Forward-declare libunwind's typedef'd unw_cursor_t:
+struct unw_cursor;
+typedef struct unw_cursor unw_cursor_t;
 
 namespace pyston {
 

src/runtime/inline/CMakeLists.txt

@@ -87,7 +87,7 @@ foreach(BC_SRC ${BC_INLINE_SRCS})
   # generate the bitcode for this file
   add_custom_command(OUTPUT ${BC_SRC_BASENAME}.bc
                      COMMAND ${BC_CXX} ${BC_DEFINES} ${BC_CXX_FLAGS} ${BC_INCLUDES} -c ${BC_SRC_FULLPATH} -o ${BC_SRC_BASENAME}.bc -emit-llvm
-                     DEPENDS ${BC_SRC_FULLPATH} clang ${PYSTON_HEADERS} ${FROM_CPYTHON_HEADERS}
+                     DEPENDS ${BC_SRC_FULLPATH} clang ${PYSTON_HEADERS} ${FROM_CPYTHON_HEADERS} libunwind
                      COMMENT "Building LLVM bitcode ${BC_SRC_BASENAME}.bc"
                      VERBATIM)
   set_property(DIRECTORY APPEND PROPERTY ADDITIONAL_MAKE_CLEAN_FILES ${BC_SRC_BASENAME}.bc)

src/runtime/stacktrace.cpp

@@ -27,6 +27,7 @@
 
 #define UNW_LOCAL_ONLY
 #include <libunwind.h>
+#undef UNW_LOCAL_ONLY
 
 namespace pyston {