CheckSubmodules won't fetch rpmalloc

[tresf]

rpmalloc uses a non-default branch and shallow clones (e.g. --depth 1) don't appear to like this.

The proposed workaround is to have CMake use a workaround when this specific error is detected.

error: no such remote ref 2e0479192b8dfb15e0084969fdf06208cffbfd09
Fetched in submodule path 'src/3rdparty/rpmalloc/rpmalloc', but it did not contain 2e0479192b8dfb15e0084969fdf06208cffbfd09. Direct fetching of that commit failed.

CMake Error at cmake/modules/CheckSubmodules.cmake:134 (MESSAGE):
  /usr/bin/git exited with status of 1
Call Stack (most recent call first):
  CMakeLists.txt:16 (INCLUDE)


-- Configuring incomplete, errors occurred!

[tresf]

Closed via d634ed3.