CMake/Deps: fix build of nasm when asciidoc and xmlto are unavailable

Create zero-byte manual page files `nasm.1` and `ndisasm.1` such that
nasm's `make install` step succeeds.

Installing nasm requires that its manual pages are built. This requires
local packages `asciidoc` and `xmlto` to be installed.

Not only does `asciidoc` pull in 110 MB of packages (itself +
dependencies), there is also no need for these manual pages. Nasm is
just used for building other dependencies, and not even part of our
precompiled libraries in SVN.

Reviewed By: sebbas

Differential Revision: https://developer.blender.org/D10396

build_files/build_environment/cmake/nasm.cmake

@@ -27,3 +27,12 @@ ExternalProject_Add(external_nasm
   INSTALL_COMMAND ${CONFIGURE_ENV} && cd ${BUILD_DIR}/nasm/src/external_nasm/ && make install
   INSTALL_DIR ${LIBDIR}/nasm
 )
+
+if(UNIX)
+  # `touch nasm.1 ndisasm.1` helps to create the manual pages files, even when
+  # local `asciidoc` and `xmlto` packages are not installed.
+  ExternalProject_Add_Step(external_nasm after_configure
+    COMMAND  ${CMAKE_COMMAND} -E touch ${BUILD_DIR}/nasm/src/external_nasm/nasm.1 ${BUILD_DIR}/nasm/src/external_nasm/ndisasm.1
+    DEPENDEES configure
+  )
+endif()