This is useful trick for in this case generating a list of all the cpp files in local directory of the makefile that is running this instruction:
SRCS := $(wildcard *.cpp)
This is GNU specific extension of the makefile system. See:
https://www.gnu.org/software/make/manual/html_node/Wildcard-Function.html
Having learned this technique I always wonder why most makefile authors maintain manual lists of source or object files in their makefiles. That is a lot of unnecessary work.
One can overcome the problem of excluding platform specific files using this trick.