add compiler support to readme and fix some compiler bugs

[rem1776]

Description
Adds a little compiler support section to the readme with a chart for whether fms builds and tests without error on different compilers. Let me know if you think it needs any more info

Also puts in two small fixes for compiler bugs:

• one line fix for failures coming from the mpp_checksum test with more recent gnu versions
• fixes a compilation error with intel's llvm based c compiler (var declarations in for loops typically aren't allowed in c, not sure why the other compilers allowed it)

How Has This Been Tested?
amd box + on gaea

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• make distcheck passes