Since openjpeg doesn't seem to ship with a .pc file, provide an alternative.

--- a/ojp2_cppflags.sh	1970-01-01 01:00:00.000000000 +0100
+++ b/ojp2_cppflags.sh	2014-09-13 22:56:38.842418777 +0200
@@ -0,0 +1,7 @@
+#!/bin/sh
+
+# Return the preprocessor flags to link against openjpeg.
+
+cpppath=$(echo ${NIX_STORE}/*-openjpeg-*/include/openjpeg-*)
+
+echo -I$cpppath

--- a/ojp2_ldflags.sh	1970-01-01 01:00:00.000000000 +0100
+++ b/ojp2_ldflags.sh	2014-09-13 22:56:38.842418777 +0200
@@ -0,0 +1,7 @@
+#!/bin/sh
+
+# Return the linker flags to link against openjpeg.
+
+ldpath=$(echo ${NIX_STORE}/*-openjpeg-*/lib)
+
+echo -L$ldpath -lopenjp2

Make use of the above alternatives, compile with gcc.

--- a/Makerules	2014-09-14 09:13:40.729149860 +0200
+++ b/Makerules	2014-09-14 09:17:06.425156595 +0200
@@ -75,12 +75,14 @@
 
 SYS_FREETYPE_CFLAGS = $(shell pkg-config --cflags freetype2)
 SYS_FREETYPE_LIBS = $(shell pkg-config --libs freetype2)
-SYS_OPENJPEG_CFLAGS = $(shell pkg-config --cflags libopenjp2)
-SYS_OPENJPEG_LIBS = $(shell pkg-config --libs libopenjp2)
+SYS_OPENJPEG_CFLAGS = $(shell ./ojp2_cppflags.sh)
+SYS_OPENJPEG_LIBS = $(shell ./ojp2_ldflags.sh)
 SYS_JBIG2DEC_LIBS = -ljbig2dec
 SYS_JPEG_LIBS = -ljpeg
 SYS_ZLIB_LIBS = -lz
 
+CC = gcc
+
 endif
 
 # The following section is an example of how to simply do cross-compilation

Remove the -x11 from the built binaries, since X11 is implied on GNU. (This
might change when Wayland gets more popular)

--- a/Makefile	2014-06-10 17:09:28.000000000 +0200
+++ b/Makefile	2014-09-14 09:57:10.381235299 +0200
@@ -255,7 +255,7 @@
 	$(LINK_CMD)
 
 ifeq "$(HAVE_X11)" "yes"
-MUVIEW_X11 := $(OUT)/mupdf-x11
+MUVIEW_X11 := $(OUT)/mupdf
 MUVIEW_X11_OBJ := $(addprefix $(OUT)/platform/x11/, x11_main.o x11_image.o pdfapp.o)
 $(MUVIEW_X11_OBJ) : $(FITZ_HDR) $(PDF_HDR)
 $(MUVIEW_X11) : $(MUPDF_LIB) $(THIRD_LIBS)
@@ -263,7 +263,7 @@
 	$(LINK_CMD) $(X11_LIBS)
 
 ifeq "$(HAVE_CURL)" "yes"
-MUVIEW_X11_CURL := $(OUT)/mupdf-x11-curl
+MUVIEW_X11_CURL := $(OUT)/mupdf-curl
 MUVIEW_X11_CURL_OBJ := $(addprefix $(OUT)/platform/x11/curl/, x11_main.o x11_image.o pdfapp.o curl_stream.o)
 $(MUVIEW_X11_CURL_OBJ) : $(FITZ_HDR) $(PDF_HDR)
 $(MUVIEW_X11_CURL) : $(MUPDF_LIB) $(THIRD_LIBS) $(CURL_LIB)