Add FindJeMalloc.cmake

[mathomp4]

This PR has adds FindJeMalloc.cmake for use with GEOS. This is NOT enabled by default but is being explored in runs of GEOS.

NOTE: To fully use this will need changes to ESMA_env (g5_modules):

• Updates to enable jemalloc build ESMA_env#116

as well as in CMake further on:

• Updates to enable jemalloc build GEOSgcm_App#568

Optional:

• Updates to enable jemalloc build ecbuild#7

The latter is optional as it turns out find_package() is case-sensitive. This FindJeMalloc.cmake file has a different name