https://bugs.gentoo.org/832677
https://github.com/kaniini/libucontext/issues/39
--- a/Makefile
+++ b/Makefile
@@ -17,7 +17,7 @@ endif
 LIBDIR := /lib
 INCLUDEDIR := /usr/include
 PKGCONFIGDIR := /usr/lib/pkgconfig
-CFLAGS := -ggdb3 -O2 -Wall
+CFLAGS ?= -ggdb3 -O2 -Wall
 CPPFLAGS := -Iinclude -Iarch/${ARCH} -Iarch/common
 EXPORT_UNPREFIXED := yes
 FREESTANDING := no