cmake: remove wide boost include

Replacing it with appending `Boost::headers` to `Boost_LIBRARIES` - this target is constructed both for Boost config and FindBoost and just references `Boost_INCLUDE_DIRS`, so it will automatically propagate to all `Boost_LIBRARIES` consumers. This is only needed for FindBoost users though.

cmake/CMakeLists.txt

@@ -389,7 +389,12 @@ if(USE_MMAP)
 endif()
 
 find_package(Boost REQUIRED COMPONENTS ${BOOST_COMPONENTS})
+# This is only needed for FindBoost, when libraries are just paths to the library files on the disk.
+# If Boost is found using cmake config, then all its Boost_LIBRARIES are targets
+# and automatically depend on the headers.
+list(APPEND Boost_LIBRARIES Boost::headers)
 message(STATUS "Boost include files found in ${Boost_INCLUDE_DIRS}")
+message(STATUS "Boost libraries found in ${Boost_LIBRARIES}")
 
 if(COLLADA_SUPPORT)
     find_package(OpenCOLLADA REQUIRED)
@@ -490,8 +495,6 @@ else()
     endif()
 endif(MSVC)
 
-include_directories(${Boost_INCLUDE_DIRS})
-
 if(NOT SCHEMA_VERSIONS)
     if(WASM_BUILD)
         # super arbitrarily try to keep size down at least a little bit