[CMake] Unnecessary rebuild of ExternalProject_Add target

[Jack Smith]

Is there something that I am missing that will prevent make from *always*
calling autogen and configure even though the target has been built and has
not been changed ?

Here is the offending project.

## Grub boot loader
set(PROJECT_NAME grub)
set(PROJECT_URL git://git.savannah.gnu.org/grub.git)
set(PROJECT_DEPENDS bison)
set(PROJECT_CONFIGURE_COMMAND ./autogen.sh && ./configure)


ExternalProject_Add(
   ${PROJECT_NAME}
   DEPENDS ${PROJECT_DEPENDS}
   GIT_REPOSITORY ${PROJECT_URL}
   CONFIGURE_COMMAND  ${PROJECT_CONFIGURE_COMMAND}
   BUILD_COMMAND make -j 2
   BUILD_IN_SOURCE 1 # What's up with that !
   CMAKE_ARGS -DCMAKE_TOOLCHAIN_FILE=${CMAKE_TOOLCHAIN_FILE}
   INSTALL_COMMAND ""
)
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