Drop the externals directory

Nix now requires SQLite and bzip2 to be pre-installed.  SQLite is
detected using pkg-config.  We required DBD::SQLite anyway, so
depending on SQLite is not a big problem.

The --with-bzip2, --with-openssl and --with-sqlite flags are gone.

src/bsdiff-4.3/Makefile.am

@@ -4,10 +4,10 @@ libexec_PROGRAMS = bsdiff bspatch
 
 bsdiff_SOURCES = bsdiff.c
 
-bsdiff_LDADD = ${bzip2_lib}
+bsdiff_LDADD = -lbz2
 
 bspatch_SOURCES = bspatch.c
 
-bspatch_LDADD = ${bzip2_lib}
+bspatch_LDADD = -lbz2
 
-AM_CFLAGS = -O3 ${bzip2_include} ${bsddiff_compat_include}
+AM_CFLAGS = -O3 ${bsddiff_compat_include}