make: add INSTALL_STRIP option variable

make: add INSTALL_STRIP option variable

Add $(INSTALL_STRIP), which allows passing stripping options to
$(INSTALL).

For this to work, installing executables must be split to installing
compiled binaries and scripts portions, since $(INSTALL_STRIP) is only
meaningful to the former.

Users can set this variable depending on their system. For example,
Linux users can use `-s --strip-program=strip`, while FreeBSD users can
simply set to `-s` and choose strip program with $STRIPBIN.

[original outline by Đoàn Trần Công Danh]

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>