Added Makefile for Linux

Added a Makefile to compile the project Engineer/ from Linux based systems
using mcs. The list of files to be compiled is parsed from the project
file Engineer.csproj and libraries are hardcoded and so is their location
(the default path is the steam install of KSP).
Fixed warning CS0219 at EngineerTools.cs:50,20

    The variable 'decimalMaskChar' is assigned but its value is never used

The warning was related to an improper variable name in duplicate code
within the number-to-string function SimpleFormatter method use to display
human-readable values. The variable in question triggers the printing of
digits after the decimal mark and could be modified to change the number
of digits to be shown. The two behaviours seem to have been separated
at one point and the trigger was deactivated. Comments have been added
in the relevant code section.
Minor: fixed warning CS1717 (x3) in Rendezvous.cs

    Assignment made to same variable; did you mean to assign something else?

Variables named 'altitude' and 'bodyPosition' were double-assigned at
declaration. The slip was most likely introduced by automatic variable
renaming.
Minor: fixed warning CS0219 in Simulation.cs

    The variable 'engineDecoupled' is assigned but its value is never used