[libsigcplusplus] Add a file about build systems.

[Murray Cumming <murrayc src gnome org>]

commit d9fac9559bc7c636a8a6562787b5a775c1fe0377
Author: Murray Cumming <murrayc murrayc com>
Date:   Sun Mar 13 20:41:12 2016 +0100

    Add a file about build systems.
    
    Otherwise people will wonder whether we use autotools or CMake as our
    main build system.

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+# Building libsigc++
+
+On Linux, you would ideally get libsigc++ from your distro's official package.
+However, this is about building from source.
+
+## autotools
+
+libsigc++ uses autotools as its main build system,
+so you can build or install with the regular configure/make steps.
+For instance:
+  ./configure --prefix=/opt/something
+then
+  make
+or
+  make install
+You can build the examples and tests, and run the tests, like so:
+  make check
+We create the tarball release like so:
+  make distcheck
+
+
+## CMake
+
+You may also build libsigc++ with CMake, though it's fairly experimental for now.
+For instance,
+  cmake -DCMAKE_INSTALL_PREFIX:PATH=/opt/something .
+then
+  make
+or
+  make install
+You can run the tests like so:
+  make test
+
+
+## Microsoft Visual C++
+
+We provide MSVC++ projects. They might work.
+Pleaes report specific bugs if they don't.