Solaris 10 requires -lposix4 to build successfully on gcc. I only have a
Sparc machine to test with on Solaris 10, but this change leaves
OpenSolaris x86 in a usable state w/ gnu-generic.

I am of the belief that this change should fix Solaris 10 on Sparc, but
will leave other Solaris architectures as is. If someone has an x86
Solaris 10 machine to test on, they may add x86-solaris-gcc to
libvpx/configure and give it a go.

Change-Id: I17a282028bb4d3e9fd8764159f95665160f7b62a