install: preserve the header time stamps
Currently when we install headers, their timestamp is the time of installation, not the time of compilation. As a consequence, we recompile the precompiled headers...
bash-3.2$ VCSN_DEBUG=1 VCSN_VERBOSE=4 /opt/gostai/bin/vcsn run tests/demo/operators
run: vcsn compile -shared '/Users/akim/.vcsn/plugins/contexts/context<letterset<char_letters>, q>.cc'
[...]
/opt/gostai/libexec/vcsn/vcsn-compile: checking whether compilation is needed
/opt/gostai/libexec/vcsn/vcsn-compile: compilation needed: dependency is more recent than output
output: 2017-03-18 18:04:28: /Users/akim/.vcsn/plugins/contexts/context<letterset<char_letters>, q>.so
dependency: 2017-03-18 18:10:49: /opt/gostai/include/vcsn/alphabets/char.hh