Allow building of just glxtrace library for ebuilds without GUI tools.

Updated from 3.0 version of patch to apply against 4.0.

--- apitrace/CMakeLists.txt
+++ apitrace/CMakeLists.txt
@@ -30,6 +30,7 @@ set (ENABLE_CLI true CACHE BOOL "Enable command Line interface.")
 
 set (ENABLE_EGL true CACHE BOOL "Enable EGL support.")
 
+option (BUILD_LIB_ONLY "Build the glxtrace library only" OFF)
 
 ##############################################################################
 # Find dependencies
@@ -248,6 +255,8 @@ endif ()
 # - The packaged versions QJson are very old, and do not support NaN/Infinity.
 # - To make it easier to build the GUI on Windows and MacOSX, as there are no
 # binaries at all.
+if (NOT BUILD_LIB_ONLY)
+
 if (QT4_FOUND)
     add_definitions (-DQJSON_EXPORT=)
     add_subdirectory (thirdparty/qjson)
@@ -257,6 +266,8 @@ if (QT4_FOUND)
     set (QJSON_FOUND TRUE)
 endif ()
 
+endif (NOT BUILD_LIB_ONLY)
+
 # We use bundled headers for all Khronos APIs, to guarantee support for both
 # OpenGL and OpenGL ES at build time, because the OpenGL and OpenGL ES 1 APIs
 # are so intertwined that conditional compilation extremely difficult. This
--- apitrace/retrace/CMakeLists.txt
+++ apitrace/retrace/CMakeLists.txt
@@ -11,6 +11,8 @@ include_directories (
 
 add_definitions (-DRETRACE)
 
+if (NOT BUILD_LIB_ONLY)
+
 add_custom_command (
     OUTPUT glretrace_gl.cpp
     COMMAND ${PYTHON_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/glretrace.py > ${CMAKE_CURRENT_BINARY_DIR}/glretrace_gl.cpp
@@ -259,3 +261,5 @@ if (WIN32)
 
     install (TARGETS d3dretrace RUNTIME DESTINATION bin)
 endif ()
+
+endif (NOT BUILD_LIB_ONLY)