replace unix with is_unix to avoid intel fpp collisions

[perazz]

Straightforward variable name change to solve #893.

Builds fine now:

root@7ce4928f525f:~/fpm# fpmx build --verbose
 <INFO> BUILD_NAME: build/ifort
 <INFO> COMPILER:  ifort
 <INFO> C COMPILER:  icc
 <INFO> CXX COMPILER: icpc
 <INFO> COMPILER OPTIONS:
 -fpp -warn all -check all -error-limit 1 -O0 -g -assume byterecl -traceback
 <INFO> C COMPILER OPTIONS:
 <INFO> CXX COMPILER OPTIONS:
 <INFO> LINKER OPTIONS:
 <INFO> INCLUDE DIRECTORIES:  []
[100%] Project compiled successfully.
root@7ce4928f525f:~/fpm#

[perazz]

Thank you @awvwgk. I will wait for another day or so and merge if there are no further comments.

[arteevraina]

👍

[perazz]

Thank you both @awvwgk @arteevraina, I believe I can merge this now.