CMakeLists: Don't dump bochs' includes into the top-level directory

Instead add it to the target interface. This way, only libraries that
link in bochs will see its include directories.

Externals/Bochs_disasm/CMakeLists.txt

@@ -1,10 +1,18 @@
-set(SRCS dis_decode.cc
-         dis_groups.cc
-         resolve.cc
-         syntax.cc)
+add_library(bdisasm STATIC
+  dis_decode.cc
+  dis_groups.cc
+  resolve.cc
+  syntax.cc
+)
 
-if(WIN32)
-    set(SRCS ${SRCS} stdafx.cc)
-endif(WIN32)
+if (WIN32)
+  target_sources(bdisasm
+  PRIVATE
+    stdafx.cc
+  )
+endif()
 
-add_library(bdisasm STATIC ${SRCS})
+target_include_directories(bdisasm
+PUBLIC
+  ${CMAKE_CURRENT_SOURCE_DIR}
+)