configure.ac: Add --enable-{client,server,examples}

These default respectively to yes, yes, no.

src/examples/Makefile.am

@@ -1,7 +1,9 @@
 AM_CPPFLAGS = $(protobuf_CFLAGS)
 AM_CXXFLAGS = $(WARNING_CXXFLAGS) $(PICKY_CXXFLAGS) -fno-default-inline -pipe
 
-noinst_PROGRAMS = encrypt decrypt ntester parse termemu benchmark
+if BUILD_EXAMPLES
+  noinst_PROGRAMS = encrypt decrypt ntester parse termemu benchmark
+endif
 
 encrypt_SOURCES = encrypt.cc
 encrypt_CPPFLAGS = -I$(srcdir)/../crypto

src/frontend/Makefile.am

@@ -9,7 +9,15 @@ else
   LDADD += $(STDDJB_LDFLAGS)
 endif
 
-bin_PROGRAMS = mosh-client mosh-server
+bin_PROGRAMS =
+
+if BUILD_CLIENT
+  bin_PROGRAMS += mosh-client
+endif
+
+if BUILD_SERVER
+  bin_PROGRAMS += mosh-server
+endif
 
 mosh_client_SOURCES = mosh-client.cc stmclient.cc stmclient.h terminaloverlay.cc terminaloverlay.h
 mosh_server_SOURCES = mosh-server.cc