diff --git a/sys/dshowsrcwrapper/BUILD.txt b/sys/dshowsrcwrapper/BUILD.txt
index 80a6f46674..994b9f4743 100644
--- a/sys/dshowsrcwrapper/BUILD.txt
+++ b/sys/dshowsrcwrapper/BUILD.txt
@@ -20,6 +20,9 @@ First build the SDK "baseclasses" sample; they should have been
 installed in <SDK>/Samples/multimedia/directshow/baseclasses. Just
 open the SLN and build both Debug_MBCS and Release_MBCS.
 
+If you get a build error "ctype.h: no such file or directory", try
+retargetting the solution from the Project menu.
+
 Then go to sys/dshowsrcwrapper and use CMake to generate the project
 file for your chosen version of Visual Studio. There are two influential
 variables:
diff --git a/sys/dshowsrcwrapper/CMakeLists.txt b/sys/dshowsrcwrapper/CMakeLists.txt
index 2e0126698d..57c3114fdd 100644
--- a/sys/dshowsrcwrapper/CMakeLists.txt
+++ b/sys/dshowsrcwrapper/CMakeLists.txt
@@ -24,6 +24,9 @@ INCLUDE_DIRECTORIES(
   ${GST_INSTALL_BASE}/lib/glib-2.0/include
   ${GST_INSTALL_BASE}/lib/gstreamer-1.0/include
   ${SDK_INSTALL_BASE}/Samples/multimedia/directshow/baseclasses
+
+  # When building from the Cerbero build dir, this is where config.h is.
+  ${PROJECT_SOURCE_DIR}/../../_builddir
 )
 
 IF ("${CMAKE_GENERATOR}" MATCHES "Win64")