in Makefile not allowed duplication of installed files.
without this fix "sudo make install" will show something like this:
/usr/bin/install: just created file «/usr/share/postgresql/9.5/extension/my_cool_c_extension--0.0.1.sql» will be not copied to «.//sql/my_cool_c_extension--0.0.1.sql»
make: *\ [install] Error 1
in Makefile not allowed duplication of installed files. without this fix "sudo make install" will show something like this: /usr/bin/install: just created file «/usr/share/postgresql/9.5/extension/my_cool_c_extension--0.0.1.sql» will be not copied to «.//sql/my_cool_c_extension--0.0.1.sql» make: *\ [install] Error 1