libpqxx: needs at least gcc 4.8 for c++11 features

This package uses autoconf (AX_CXX_COMPILE_STDCXX_11) to validate C++11
support. The test code uses an auto static data member for validation;
however, support for this only exists in GCC 4.8+ [1]. This causes the
configuration script to generated the following errors:

    checking whether .../host/bin/powerpc-ctng_e500v2-linux-gnuspe-g++ accepts -g... yes
    checking whether .../host/bin/powerpc-ctng_e500v2-linux-gnuspe-g++ supports C++11 features by default... no
    checking whether .../host/bin/powerpc-ctng_e500v2-linux-gnuspe-g++ supports C++11 features with -std=c++11... no
    checking whether .../host/bin/powerpc-ctng_e500v2-linux-gnuspe-g++ supports C++11 features with +std=c++11... no
    checking whether .../host/bin/powerpc-ctng_e500v2-linux-gnuspe-g++ supports C++11 features with -h std=c++11... no
    checking whether .../host/bin/powerpc-ctng_e500v2-linux-gnuspe-g++ supports C++11 features with -std=c++0x... no
    checking whether .../host/bin/powerpc-ctng_e500v2-linux-gnuspe-g++ supports C++11 features with +std=c++0x... no
    checking whether .../host/bin/powerpc-ctng_e500v2-linux-gnuspe-g++ supports C++11 features with -h std=c++0x... no

Bumping the required GCC version of this package to at least v4.8.

Fixes:

    http://autobuild.buildroot.net/results/fae9ccc37c31c6d1852d0dc11d43f9c6ecbf1fe4/

[1]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55003



Signed-off-by: James Knight <james.knight@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>