base64 output includes some CR and the shell seems to do word
splitting... not fun.

Index: Makefile
--- Makefile.orig
+++ Makefile
@@ -15,7 +15,7 @@ BUILD_OPTS?=-trimpath
 # ignore environment variable
 GO_LDFLAGS:=
 GO_LDFLAGS+=-X main.Version=$(VERSION)
-GO_LDFLAGS+=-X main.Flags=$$(echo -- $(GOFLAGS) | base64 | tr -d '\n')
+GO_LDFLAGS+=-X main.Flags=$$(echo -- $(GOFLAGS) | base64 | tr -d '\r\n')
 GO_LDFLAGS+=-X git.sr.ht/~rjarry/aerc/config.shareDir=$(SHAREDIR)
 GO_LDFLAGS+=-X git.sr.ht/~rjarry/aerc/config.libexecDir=$(LIBEXECDIR)
 GO_LDFLAGS+=$(GO_EXTRA_LDFLAGS)