Statically link more libraries in on Linux

[smashwilson]

I'm not doing anything special yet to limit external dependencies on Linux, so the binaries depend on a bunch of stuff:

[==] Dynamic dependencies of the GPG binaries:
[==] /home/travis/build/atom/squeegpg-native/build/gnupg/bin/gpg
	linux-vdso.so.1 =>  (0x00007fff019f1000)
	libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007fee8ee9b000)
	libbz2.so.1.0 => /lib/x86_64-linux-gnu/libbz2.so.1.0 (0x00007fee8ec8b000)
	libsqlite3.so.0 => /usr/lib/x86_64-linux-gnu/libsqlite3.so.0 (0x00007fee8e9d2000)
	libreadline.so.6 => /lib/x86_64-linux-gnu/libreadline.so.6 (0x00007fee8e78c000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fee8e3c3000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fee8e1a5000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007fee8dfa1000)
	libtinfo.so.5 => /lib/x86_64-linux-gnu/libtinfo.so.5 (0x00007fee8dd78000)
	/lib64/ld-linux-x86-64.so.2 (0x00007fee8f0b4000)
[==] /home/travis/build/atom/squeegpg-native/build/gnupg/bin/gpg-agent
	linux-vdso.so.1 =>  (0x00007ffe7db4b000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f7c08ca8000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f7c088df000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f7c08ec6000)