Fix $filter and Msys/Cygwin

- switched the patter and input of $filter into the right places
- added pattern wildcard to MSYS_NT & CYGWIN_NT as they change with windows versions
- correctly identify MSYS2, even in an env like MINGW64

tests/fuzz/Makefile

@@ -17,7 +17,7 @@ LDFLAGS ?=
 ARFLAGS ?=
 LIB_FUZZING_ENGINE ?= libregression.a
 PYTHON ?= python
-ifeq ($(shell uname), Darwin)
+ifeq ($(shell sh -c 'MSYSTEM="MSYS" uname') , Darwin)
 	DOWNLOAD?=curl -L -o
 else
 	DOWNLOAD?=wget -O