libgit2: add as a separated component instead of embedded one in 'rugged' gem.

so that we can explicitly specify which libraries should be / should
not be used and set RPATH appropriately, otherwise existing system
libraries may be used and the result depends on the environment where
rugged is built.

component/libgit2/buildout.cfg

+[buildout]
+extends =
+  ../cmake/buildout.cfg
+  ../curl/buildout.cfg
+  ../openssl/buildout.cfg
+  ../pkgconfig/buildout.cfg
+  ../zlib/buildout.cfg
+
+[libgit2]
+# The version here heavily depends on the version of 'rugged' gem that
+# is used in gitlab.
+recipe = slapos.recipe.cmmi
+url = https://github.com/libgit2/libgit2/archive/v0.24.0.tar.gz
+md5sum = 8cabf04502d7203793b32f47ca410ae3
+location = ${buildout:parts-directory}/${:_buildout_section_name_}
+configure-command = ${cmake:location}/bin/cmake
+configure-options =
+  -DCMAKE_INSTALL_PREFIX=${:location}
+  -DCURL=1
+  -DUSE_GSSAPI=0
+  -DUSE_OPENSSL=1
+  -DUSE_SSH=0
+  -DCMAKE_INSTALL_RPATH=${curl:location}/lib:${openssl:location}/lib:${zlib:location}/lib
+environment =
+  CMAKE_INCLUDE_PATH=${curl:location}/include:${openssl:location}/include:${zlib:location}/include
+  CMAKE_LIBRARY_PATH=${curl:location}/lib:${openssl:location}/lib:${zlib:location}/lib
+  LDFLAGS=-L${curl:location}/lib
+  PATH=${pkgconfig:location}/bin:%(PATH)s
+  PKG_CONFIG_PATH=${curl:location}/lib/pkgconfig:${openssl:location}/lib/pkgconfig