Add AVX2 prime check with cross-platform Makefile

Trial division by 6k+/-1 using double-precision AVX2 lanes so the quotient is exact for 32-bit n. Makefile builds on Linux and Windows (MinGW gcc).

Makefile

@@ -0,0 +1,36 @@
+# Builds prime_avx2 on Linux (gcc) and Windows (MinGW-w64 / MSYS2 gcc).
+#   make          build
+#   make run      build and run
+#   make clean    remove build output
+
+CC      ?= gcc
+CFLAGS  ?= -O3 -mavx2 -Wall -Wextra
+LDLIBS  += -lm
+
+ifeq ($(OS),Windows_NT)
+  EXE   := .exe
+  RM    := del /Q /F
+  # make on Windows may use cmd.exe, which needs backslashes
+  RUN   := .\prime_avx2$(EXE)
+  RMTARGET := prime_avx2$(EXE)
+else
+  EXE   :=
+  RM    := rm -f
+  RUN   := ./prime_avx2
+  RMTARGET := prime_avx2
+endif
+
+TARGET := prime_avx2$(EXE)
+
+.PHONY: all run clean
+
+all: $(TARGET)
+
+$(TARGET): prime_avx2.c
+	$(CC) $(CFLAGS) $< -o $@ $(LDLIBS)
+
+run: $(TARGET)
+	$(RUN)
+
+clean:
+	-$(RM) $(RMTARGET)