Fixed it to compile with GCC 11

Makefile

 all:
 	tar xf efivar-37.tar.bz2
 	patch -Np1 -d efivar-37 < efivar-37-gcc9_fix.patch
+	patch -Np1 -d efivar-37 < efivar-37-no_lto.patch
+	patch -Np1 -d efivar-37 < efivar-37-no_werror.patch
 	$(MAKE) -C efivar-37 libdir=/usr/lib bindir=/usr/bin mandir=/usr/share/man includedir=/usr/include
 	$(MAKE) -C efivar-37 libdir=/usr/lib bindir=/usr/bin mandir=/usr/share/man includedir=/usr/include install
 	rm -rf efivar-37

efivar-37-no_lto.patch

+diff -uNr efivar-37/efivar.spec efivar-37-no_lto/efivar.spec
+--- efivar-37/efivar.spec	2018-12-06 02:54:41.000000000 +1030
++++ efivar-37-no_lto/efivar.spec	2022-11-03 13:03:45.134677129 +1030
+@@ -38,7 +38,7 @@
+ git config --unset user.name
+ 
+ %build
+-make LIBDIR=%{_libdir} BINDIR=%{_bindir} CFLAGS="$RPM_OPT_FLAGS -flto" LDFLAGS="$RPM_LD_FLAGS -flto"
++make LIBDIR=%{_libdir} BINDIR=%{_bindir} CFLAGS="$RPM_OPT_FLAGS" LDFLAGS="$RPM_LD_FLAGS"
+ 
+ %install
+ rm -rf $RPM_BUILD_ROOT
+diff -uNr efivar-37/efivar.spec.in efivar-37-no_lto/efivar.spec.in
+--- efivar-37/efivar.spec.in	2018-12-06 02:47:42.000000000 +1030
++++ efivar-37-no_lto/efivar.spec.in	2022-11-03 13:03:34.425646302 +1030
+@@ -38,7 +38,7 @@
+ git config --unset user.name
+ 
+ %build
+-make LIBDIR=%{_libdir} BINDIR=%{_bindir} CFLAGS="$RPM_OPT_FLAGS -flto" LDFLAGS="$RPM_LD_FLAGS -flto"
++make LIBDIR=%{_libdir} BINDIR=%{_bindir} CFLAGS="$RPM_OPT_FLAGS" LDFLAGS="$RPM_LD_FLAGS"
+ 
+ %install
+ rm -rf $RPM_BUILD_ROOT
+diff -uNr efivar-37/Make.defaults efivar-37-no_lto/Make.defaults
+--- efivar-37/Make.defaults	2018-12-06 02:47:42.000000000 +1030
++++ efivar-37-no_lto/Make.defaults	2022-11-03 13:03:52.334697774 +1030
+@@ -16,7 +16,7 @@
+ CC	:= $(if $(filter default,$(origin CC)),$(CROSS_COMPILE)$(COMPILER),$(CC))
+ CCLD_FOR_BUILD	?= $(CC_FOR_BUILD)
+ CCLD	:= $(if $(filter undefined,$(origin CCLD)),$(CC),$(CCLD))
+-OPTIMIZE ?= -O2 -flto
++OPTIMIZE ?= -O2
+ CFLAGS	?= $(OPTIMIZE) -g3
+ CFLAGS	:= $(CFLAGS)
+ LDFLAGS ?=

efivar-37-no_werror.patch

+diff -uNr efivar-37/gcc.specs efivar-37-no_werror/gcc.specs
+--- efivar-37/gcc.specs	2018-12-06 02:47:42.000000000 +1030
++++ efivar-37-no_werror/gcc.specs	2022-11-03 13:24:10.310188717 +1030
+@@ -2,7 +2,7 @@
+ + -D_GNU_SOURCE
+ 
+ *efivar_cpp_options:
+- -Werror -Wall -std=gnu11 -Wextra
++ -Wall -std=gnu11 -Wextra
+ 
+ *cpp_options:
+ + %(efivar_cpp_options)