Building libsoup-2.28.2 on OS X 10.4:

/bin/sh ../libtool  --tag=CC   --mode=link gcc  -Os             -Wall -Wstrict-prototypes -Wmissing-declarations                -Wmissing-prototypes -Wnested-externs -Wpointer-arith           -Wdeclaration-after-statement -Wformat=2 -Winit-self            -Waggregate-return -Wmissing-format-attribute -Wmissing-include-dirs -Wundef -D_REENTRANT -version-info 4:0:3 -no-undefined -L/sw/lib -o libsoup-2.4.la -rpath /sw/lib soup-marshal.lo soup-enum-types.lo soup-address.lo soup-auth.lo soup-auth-basic.lo soup-auth-digest.lo soup-auth-ntlm.lo soup-auth-domain.lo soup-auth-domain-basic.lo soup-auth-domain-digest.lo soup-auth-manager.lo soup-auth-manager-ntlm.lo soup-coding.lo soup-coding-gzip.lo soup-connection.lo soup-content-decoder.lo soup-content-sniffer.lo soup-cookie.lo soup-cookie-jar.lo soup-cookie-jar-text.lo soup-date.lo soup-form.lo soup-gnutls.lo soup-headers.lo soup-logger.lo soup-message.lo soup-message-body.lo soup-message-client-io.lo soup-message-headers.lo soup-message-io.lo soup-message-queue.lo soup-message-server-io.lo soup-method.lo soup-misc.lo soup-multipart.lo soup-nossl.lo soup-password-manager.lo soup-path-map.lo soup-proxy-resolver.lo soup-proxy-resolver-static.lo soup-proxy-uri-resolver.lo soup-server.lo soup-session.lo soup-session-async.lo soup-session-feature.lo soup-session-sync.lo soup-socket.lo soup-status.lo soup-uri.lo soup-value-utils.lo soup-xmlrpc.lo -L/sw/lib -lgthread-2.0 -lgio-2.0 -lgobject-2.0 -lgmodule-2.0 -lglib-2.0 -L/sw/lib -lxml2  -L/sw/lib -lgnutls -L/sw/lib -lgcrypt -lgpg-error  
libtool: link: gcc -dynamiclib  -o .libs/libsoup-2.4.1.dylib  .libs/soup-marshal.o .libs/soup-enum-types.o .libs/soup-address.o .libs/soup-auth.o .libs/soup-auth-basic.o .libs/soup-auth-digest.o .libs/soup-auth-ntlm.o .libs/soup-auth-domain.o .libs/soup-auth-domain-basic.o .libs/soup-auth-domain-digest.o .libs/soup-auth-manager.o .libs/soup-auth-manager-ntlm.o .libs/soup-coding.o .libs/soup-coding-gzip.o .libs/soup-connection.o .libs/soup-content-decoder.o .libs/soup-content-sniffer.o .libs/soup-cookie.o .libs/soup-cookie-jar.o .libs/soup-cookie-jar-text.o .libs/soup-date.o .libs/soup-form.o .libs/soup-gnutls.o .libs/soup-headers.o .libs/soup-logger.o .libs/soup-message.o .libs/soup-message-body.o .libs/soup-message-client-io.o .libs/soup-message-headers.o .libs/soup-message-io.o .libs/soup-message-queue.o .libs/soup-message-server-io.o .libs/soup-method.o .libs/soup-misc.o .libs/soup-multipart.o .libs/soup-nossl.o .libs/soup-password-manager.o .libs/soup-path-map.o .libs/soup-proxy-resolver.o .libs/soup-proxy-resolver-static.o .libs/soup-proxy-uri-resolver.o .libs/soup-server.o .libs/soup-session.o .libs/soup-session-async.o .libs/soup-session-feature.o .libs/soup-session-sync.o .libs/soup-socket.o .libs/soup-status.o .libs/soup-uri.o .libs/soup-value-utils.o .libs/soup-xmlrpc.o   -L/sw/lib /sw/lib/libgthread-2.0.dylib /sw/lib/libgio-2.0.dylib /sw/lib/libgobject-2.0.dylib /sw/lib/libgmodule-2.0.dylib /sw/lib/libglib-2.0.dylib /sw/lib/libxml2.dylib /sw/lib/libgnutls.dylib /sw/lib/libgcrypt.dylib /sw/lib/libgpg-error.dylib    -install_name  /sw/lib/libsoup-2.4.1.dylib -compatibility_version 5 -current_version 5.0 -Wl,-single_module
ld: Undefined symbols:
_deflate
_deflateEnd
_deflateInit2_
_inflate
_inflateEnd
_inflateInit2_
/usr/libexec/gcc/powerpc-apple-darwin8/4.0.1/libtool: internal link edit command failed
make[3]: *** [libsoup-2.4.la] Error 1

soup-coding-gzip.c uses zlib so need to link libz to resolve those symbols. Adding -lz to libsoup/Makefile.am:libsoup_2_4_la_LDFLAGS fixes it.