diff --git a/Make.inc b/Make.inc
index 35eda23..ad38fab 100644
--- a/Make.inc
+++ b/Make.inc
@@ -3,7 +3,7 @@
 # Default build rule for any Makefile in this project: all
 default: all
 
-OS := $(shell uname)
+OS ?= $(shell uname)
 # Do not forget to bump SOMINOR when changing VERSION,
 # and SOMAJOR when breaking ABI in a backward-incompatible way
 VERSION = 0.8.0
@@ -168,7 +168,7 @@ ifneq ($(filter $(ARCH),i387 amd64),)
 # linux x86_64, for instance, `long double` is 80 bits wide, whereas on macOS aarch64,
 # `long double` is the same as `double`.
 LONG_DOUBLE_NOT_DOUBLE := 1
-else ifeq ($(ARCH), aarch64 riscv64)
+else ifneq ($(filter $(ARCH),aarch64 riscv64),)
 ifeq ($(filter $(OS),Darwin WINNT),)
 LONG_DOUBLE_NOT_DOUBLE := 1
 endif