Add use_system_libopenjpeg2 build argument

This cl adds the possibility to build pdfium on Linux using a system
provided copy of libopenjpeg2, as it is preferred by many distros.

Change-Id: Id2805ffbf7b75678c2c1a73469ae9dfb9278f97b
Reviewed-on: https://pdfium-review.googlesource.com/c/50750
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/core/fxcodec/BUILD.gn b/core/fxcodec/BUILD.gn
index e09b7a6..66cfd71 100644
--- a/core/fxcodec/BUILD.gn
+++ b/core/fxcodec/BUILD.gn
@@ -70,8 +70,8 @@
   configs += [ "../../:pdfium_core_config" ]
   include_dirs = []
   deps = [
-    "../../third_party:fx_libopenjpeg",
     "../../third_party:lcms2",
+    "../../third_party:libopenjpeg2",
     "../../third_party:zlib",
     "../fxcrt",
     "../fxge",