Building webkit from source

I tried to install the webkit applicaion from sources using this manual:

https://help.ubuntu.com/community/WebKit

When I type:

./autogen.sh --prefix=/usr

the following error occurs:

checking for _FILE_OFFSET_BITS value needed for large files... 64
configure: error: Compiler GCC >= 4.7 or Clang >= 3.0 is required for C++ compilation

When I type:

gcc --version

It responds:

gcc (Ubuntu/Linaro 4.7.2-11precise2) 4.7.2

Hence the error does not concern gcc version. Why the execution fails?

Edited: Previous problem is solved by updating g++ to 4.7. But the following error occurs further:

checking for GAIL... no
configure: error: Package requirements (gail-3.0 >= 3.0) were not met:

No package 'gail-3.0' found 

Consider adjusting the PKG_CONFIG_PATH environment variable if you
installed software in a non-standard prefix.

Alternatively, you may set the environment variables GAIL_CFLAGS
and GAIL_LIBS to avoid the need to call pkg-config.
See the pkg-config man page for more details.

I have apt-get install gail-3.0, but the error remains.

Edited: Also did:

apt-get install gail-3.0-dev
apt-get build-dep webkit

But further missing packages revealed: error: Package requirements (libsoup-2.4 >= 2.40.0) were not met:

Requested 'libsoup-2.4 >= 2.40.0' but version of libsoup is 2.38.1

Consider adjusting the PKG_CONFIG_PATH environment variable if you
installed software in a non-standard prefix.

Alternatively, you may set the environment variables LIBSOUP_CFLAGS
and LIBSOUP_LIBS to avoid the need to call pkg-config.
See the pkg-config man page for more details.