On a Fedora 15 x864_64 system, /usr/lib64/libX11.so.6 is not being included in the link process.
ld handily tells me that 'XConvertSelection' is defined in DSO /usr/lib64/libX11.so.6, so I need only add libX11 to LIBS.
--- Makefile.orig 2011-12-05 21:05:54.268312242 -0600
+++ Makefile 2011-12-05 20:54:46.654298126 -0600
@@ -3,7 +3,7 @@
CC = gcc
OBJ = blit.o debug.o dis.o error.o load_cmd.o load_hex.o main.o trs_mkdisk.o trs_cassette.o trs_chars.o trs_disk.o trs_hard.o trs_imp_exp.o trs_interrupt.o trs_io.o trs_memory.o trs_printer.o trs_rom1.o trs_rom3.o trs_rom4p.o trs_sdl_gui.o trs_sdl_interface.o trs_sdl_keyboard.o trs_state_save.o trs_uart.o z80.o sdltrs_main.o PasteManager.o
LINKOBJ = blit.o debug.o dis.o error.o load_cmd.o load_hex.o main.o trs_mkdisk.o trs_cassette.o trs_chars.o trs_disk.o trs_hard.o trs_imp_exp.o trs_interrupt.o trs_io.o trs_memory.o trs_printer.o trs_rom1.o trs_rom3.o trs_rom4p.o trs_sdl_gui.o trs_sdl_interface.o trs_sdl_keyboard.o trs_state_save.o trs_uart.o z80.o sdltrs_main.o PasteManager.o
-LIBS = -lSDL -lXt
+LIBS = -lSDL -lXt -lX11
INCS =
BIN = sdltrs
CFLAGS = $(INCS) -g
I had to make the same change for it to link properly under Linux Mint 17 (which presumably means the same for Ubuntu 14.04 too).
In fact, in order to effectuate compilation within an Ubuntu 14.10 enviroment I was impelled to make this modification as well.