You are not just trying to install squid, you are clearly trying to build it as well. Because you are building under Solaris, some warnings are likely owing to differences in header file from Linux. I suspect the gcc line leading up to the above contained the option -Werror (treat warnings as errors). You probably have to fix this yourself - examine pam_auth.c at line 133 - what is being initialised? What is its type? Can you see how to change it so it will compile clean? It depends how close what you have is to what pam_auth.c expects. If it's a change in signedness or even better just a const give or take, it's probably OK to patch the C code to agree with what the system provides.
The other thing you could try is removing -Werror. Warnings can point to an actual problem, but they don't always. Find -Werror in the various Makefiles and remove. Or find in the various Makefile.ins and remove, then re-run configure
