Quick fix: edit configure, at row 3235 delete -Wno-unused-result from the CPPFLAGS (found on pound mailing list).
Today I had to compile Pound reverse proxy on a CentOS 5.6 host. Initially, the configure process failed to find ssl libs, or any other lib:
$ ./configure checking for gcc... gcc checking whether the C compiler works... yes checking for C compiler default output file name... a.out checking for suffix of executables... checking whether we are cross compiling... no checking for suffix of object files... o checking whether we are using the GNU C compiler... yes checking whether gcc accepts -g... yes checking for gcc option to accept ISO C89... none needed checking for a BSD-compatible install... /usr/bin/install -c checking build system type... x86_64-unknown-linux-gnu checking host system type... x86_64-unknown-linux-gnu checking target system type... x86_64-unknown-linux-gnu configure: *** Checking for threads library and/or flags *** checking for the pthreads library -lpthreads... no checking whether pthreads work without any flags... no checking whether pthreads work with -Kthread... no checking whether pthreads work with -kthread... no checking for the pthreads library -llthread... no checking whether pthreads work with -pthread... no checking whether pthreads work with -pthreads... no checking whether pthreads work with -mthreads... no checking for the pthreads library -lpthread... no checking whether pthreads work with --thread-safe... no checking whether pthreads work with -mt... no checking for pthread-config... no configure: *** Parsing options *** configure: *** Checking for libraries *** checking for dlopen in -ldl... no checking for socket in -lsocket... no checking for hstrerror in -lresolv... no checking for BIO_new in -lcrypto... no Missing OpenSSL (-lcrypto) - aborted
After some fiddling and much googling, I found out that there was no problem with the compile environment, instead the configure script was using a particular flag, -Wno-unused-result, that is not supported by the old GCC version installed in CentOS 5.x.
You can verify by compiling any C file:
$ gcc -Wno-unused-result -o my-gcc-is-old http.c cc1: error: unrecognized command line option "-Wno-unused-result"
So, GCC is failing to compile the sample programs that configure uses to determine if a lib (or some function in a lib) is available, hence configure aborts lamenting the absence of the library.