Some demos build files support incremental build - i.e. you change one .c file and only one file is recompiled, for instance demos\STM32\RT-STM32F401C-DISCOVERY - this is proper expected behavious
Unfortunatelly a bunch of demos like demos\various\RT-Posix-Simulator are NOT incremental - any invocation of 'make' causes complete recompilation.
This is my local issue, sorry
I was using make from GNU ARM Eclipse\Build Tools
once I hit make from GNU ARM Eclipse\Build Tools and installed make from Cygwin things got fine!