fix cmake include directories at target level

prevent a "which is prefixed in the build directory" error
by providing a full path and setting only for BUILD_INTERFACE
to avoid conflict with cmake's install call below
diff --git a/double-conversion/CMakeLists.txt b/double-conversion/CMakeLists.txt
index fceedf9..c4484be 100644
--- a/double-conversion/CMakeLists.txt
+++ b/double-conversion/CMakeLists.txt
@@ -23,7 +23,7 @@
 ${headers}
 )
 
-target_include_directories(double-conversion PUBLIC ..)
+target_include_directories(double-conversion PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/..>)
 
 #
 # associates the list of headers with the library