[CMake] Patch utility is installed, but my ExternalProject build step fails to execute it

[David Brown]

I'm trying to patch and build a Makefile-based project using the
ExternalProject module. My patch command looks like this:

    PATCH_COMMAND "patch -p1 -t -N < ${CMAKE_CURRENT_SOURCE_DIR}/IntelDFP.patch"

When building, however, I get this:

    [ 10%] Performing patch step for 'IntelDFP'
    /bin/sh: 1: patch -p1 -t -N <
/home/david/Projects/db/IntelDFP.patch: not found

I'm certain that the patch utility is installed and that the patch
file path is correct. If I run that command myself, it works fine. So
why does the CMake-generated Makefile fail to execute it?