libwebp/CMake: Add <BUILD_INTERFACE> to webp incl

This way target_link_libraries(my_target PUBLIC webp) is enough,
no need to add include directories explicitly for the public headers
such as src/webp/decode.h. This matches the /usr/include/webp/decode.h
installation folder path prefix (#include "webp/decode.h").
See https://bugs.chromium.org/p/webp/issues/detail?id=532

The target_include_directories() calls for cwebp, dwebp etc. may be
unnecessary now and may be removed in another patch.

Change-Id: I2c85218920ffc35ebec23e0dd239f71e29add23b
diff --git a/CMakeLists.txt b/CMakeLists.txt
index d64f676..bd1bebb 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -283,10 +283,12 @@
   libwebp_add_stub_file(webp)
 endif()
 target_link_libraries(webp ${WEBP_DEP_LIBRARIES})
-target_include_directories(webp
-                           PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}
-                                   ${CMAKE_CURRENT_BINARY_DIR}
-                           PUBLIC $<INSTALL_INTERFACE:include>)
+target_include_directories(
+  webp
+  PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}
+          ${CMAKE_CURRENT_BINARY_DIR}
+  PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/src>
+         $<INSTALL_INTERFACE:include>)
 set_target_properties(
   webp
   PROPERTIES PUBLIC_HEADER "${CMAKE_CURRENT_SOURCE_DIR}/src/webp/decode.h;\