fix(cmake/subdirectory): fix project import by other sdk by add-subdirectory

notice: if you are cross build, please follow step
----
define SRC_DIR you sdk root dir
SRC_DIR=$($READLINK -f "`dirname $0`")
source $SRC_DIR/megbrain/scripts/cmake-build/utils/utils.sh
build_flatc $SRC_DIR/megbrain false

your build code
----

GitOrigin-RevId: ad97860f5f

cmake/FetchMegBrainVersion.cmake

@@ -7,7 +7,7 @@
 # MGB_VER_STRING: Version string
 option(MGB_FORCE_DEV_VERSION "Force -dev tag in version stamp" OFF)
 
-file (READ "${CMAKE_SOURCE_DIR}/src/core/include/megbrain/version.h" content)
+file (READ "${CMAKE_CURRENT_SOURCE_DIR}/src/core/include/megbrain/version.h" content)
 
 string (REGEX MATCH "MGB_MAJOR +([0-9]+)" _ ${content})
 set (MGB_VER_MAJOR ${CMAKE_MATCH_1})

scripts/cmake-build/utils/utils.sh

@@ -15,6 +15,7 @@ if [ $OS = "Darwin" ];then
     READLINK=greadlink
 fi
 
+PROJECT_DIR=$(dirname "${BASH_SOURCE[0]}")/../../../
 function cd_real_build_dir() {
     REAL_DIR=$($READLINK -f $1)
     echo "may alias dir: $1"
@@ -46,7 +47,7 @@ function build_flatc() {
         -DFLATBUFFERS_BUILD_FLATHASH=OFF \
         -DFLATBUFFERS_BUILD_FLATLIB=OFF \
         -DFLATBUFFERS_LIBCXX_WITH_CLANG=OFF \
-        $SRC_DIR/third_party/flatbuffers
+        ${PROJECT_DIR}/third_party/flatbuffers
 
     ${NINJA_BASE} all
     ${NINJA_BASE} install/strip