icd: build a static library

libicd was an OBJECT library because it provided the XGL entry points.  That
is not true anymore.  Make it a STATIC library.

v2:
 - update .gitignore
 - stop adding CMAKE_CURRENT_BINARY_DIR to ICD_INCLUDE_DIRS, unnecessary since
   the switch to xglLayer.h some time ago

v3: Make icd appear before ICD_LIBRARIES during linking of libXGL_i965.so.
    It looks like we can do better now that icd is a regular library, but
    leave it as is for now.
diff --git a/.gitignore b/.gitignore
index 4e4fe6f..c4509a9 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,6 +6,7 @@
 XGLConfig.h
 *.so
 *.so.*
+icd/common/libicd.a
 loader/dispatch.c
 tests/xgl_image_tests
 tests/xgl_render_tests