ExternalProject keeps rebuilding

No matter if GIT_TAG is a branch name or a fixed commit CMake keeps re-running all the steps every time I build which does take some time. Other people ran into this too.

ExternalProject_Add(ext_glbinding
  GIT_REPOSITORY https://github.com/cginternals/glbinding.git
  GIT_TAG        master
  GIT_SHALLOW    TRUE
  INSTALL_DIR    ${CMAKE_BINARY_DIR}
  CMAKE_ARGS     -DCMAKE_INSTALL_PREFIX=<INSTALL_DIR>
)

Performing download step (git clone) for 'ext_glbinding'
-- Avoiding repeated git clone, stamp file is up to date
Performing update step for 'ext_glbinding'
Current branch master is up to date.
No patch step for 'ext_glbinding'
Performing configure step for 'ext_glbinding'
...
Performing build step for 'ext_glbinding'
...
Performing install step for 'ext_glbinding'
...
 -- Up-to-date: ...
 -- Up-to-date: ...
...
Completed 'ext_glbinding'