.gitmodules: remove stale http-parser entry
Makefile.am: readd the -pedantic flag

I used to compile libneubot with -pedantic flag, but the flag was
lost when I switched libight to use autotools.
Tell configure.ac we want silent rules (is possible)

The silent rules make the build process less verbose and more
similar to the (very beautiful!) Linux kernel build process.

Starting from automake 1.11, it is possible to tell automake to
support silent rules with AM_SILENT_RULES.

Starting from automake 1.13, automake always generates Makefiles
supporting silent rules.

The way in which I request silent rules in configure.ac is such
that automake 1.11 is not required and such that silent rules are
enabled only if possibile (i.e. automake >= 1.11).

Because the default is to produce verbose rules, I've added to
README.md instructions on how to enable silent rules.

More background on silent rules here:

  https://www.flameeyes.eu/autotools-mythbuster/automake/silent.html
configure.ac: require C99 and C++11

Apparently, autoconf does not provide a check for C++11, so I
rolled out my own check, which is patterned after this:

    http://stackoverflow.com/questions/1354996/

I concluded that autoconf 2.69 does not provide a check for C++11
after I inspected the "C++ Compiler Characteristics" section of
its manual, which is available here:

    http://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/C_002b_002b-Compiler.html

To check whether this macro correctly detects a compiler that does
not implement the C++11 standard, I edited configure.ac to check for
`-std=c++12`; I can confirm that `./configure` fails in that case.
configure.ac: zap commented-out code

Should have zapped it before committing configure.ac, sorry.