polarssl: don't build empty objects

Fixes the following warnings on OSX:

/Library/Developer/CommandLineTools/usr/bin/ranlib: file: .libs/libpolarssl.a(libpolarssl_la-havege.o) has no symbols
/Library/Developer/CommandLineTools/usr/bin/ranlib: file: .libs/libpolarssl.a(libpolarssl_la-md2.o) has no symbols
/Library/Developer/CommandLineTools/usr/bin/ranlib: file: .libs/libpolarssl.a(libpolarssl_la-md4.o) has no symbols
/Library/Developer/CommandLineTools/usr/bin/ranlib: file: .libs/libpolarssl.a(libpolarssl_la-memory_buffer_alloc.o) has no symbols
/Library/Developer/CommandLineTools/usr/bin/ranlib: file: .libs/libpolarssl.a(libpolarssl_la-memory.o) has no symbols
/Library/Developer/CommandLineTools/usr/bin/ranlib: file: .libs/libpolarssl.a(libpolarssl_la-padlock.o) has no symbols
/Library/Developer/CommandLineTools/usr/bin/ranlib: file: .libs/libpolarssl.a(libpolarssl_la-pkcs11.o) has no symbols
/Library/Developer/CommandLineTools/usr/bin/ranlib: file: .libs/libpolarssl.a(libpolarssl_la-threading.o) has no symbols
/Library/Developer/CommandLineTools/usr/bin/ranlib: file: .libs/libpolarssl.a(libpolarssl_la-havege.o) has no symbols
/Library/Developer/CommandLineTools/usr/bin/ranlib: file: .libs/libpolarssl.a(libpolarssl_la-md2.o) has no symbols
/Library/Developer/CommandLineTools/usr/bin/ranlib: file: .libs/libpolarssl.a(libpolarssl_la-md4.o) has no symbols
/Library/Developer/CommandLineTools/usr/bin/ranlib: file: .libs/libpolarssl.a(libpolarssl_la-memory_buffer_alloc.o) has no symbols
/Library/Developer/CommandLineTools/usr/bin/ranlib: file: .libs/libpolarssl.a(libpolarssl_la-memory.o) has no symbols
/Library/Developer/CommandLineTools/usr/bin/ranlib: file: .libs/libpolarssl.a(libpolarssl_la-padlock.o) has no symbols
/Library/Developer/CommandLineTools/usr/bin/ranlib: file: .libs/libpolarssl.a(libpolarssl_la-pkcs11.o) has no symbols
/Library/Developer/CommandLineTools/usr/bin/ranlib: file: .libs/libpolarssl.a(libpolarssl_la-threading.o) has no symbols
Merge pull request #1759 from rubenk/polarssl-build-only-what-we-need

polarssl: don't build empty objects