Allow overriding clipboard error message helpers (gopasspw#1851)

This is useful for packaging gopass on some platforms and
a no-op on others.

RELEASE_NOTES=n/a

Signed-off-by: Dominik Schulz <dominik.schulz@gauner.org>

Makefile

@@ -9,9 +9,10 @@ GOPASS_REVISION           := $(shell cat COMMIT 2>/dev/null || git rev-parse --s
 BASH_COMPLETION_OUTPUT    := bash.completion
 FISH_COMPLETION_OUTPUT    := fish.completion
 ZSH_COMPLETION_OUTPUT     := zsh.completion
+CLIPHELPERS               ?= ""
 # Support reproducible builds by embedding date according to SOURCE_DATE_EPOCH if present
 DATE                      := $(shell date -u -d "@$(SOURCE_DATE_EPOCH)" '+%FT%T%z' 2>/dev/null || date -u '+%FT%T%z')
-BUILDFLAGS_NOPIE          := -trimpath -ldflags="-s -w -X main.version=$(GOPASS_VERSION) -X main.commit=$(GOPASS_REVISION) -X main.date=$(DATE)" -gcflags="-trimpath=$(GOPATH)" -asmflags="-trimpath=$(GOPATH)"
+BUILDFLAGS_NOPIE          := -trimpath -ldflags="-s -w -X main.version=$(GOPASS_VERSION) -X main.commit=$(GOPASS_REVISION) -X main.date=$(DATE) $(CLIPHELPERS)" -gcflags="-trimpath=$(GOPATH)" -asmflags="-trimpath=$(GOPATH)"
 BUILDFLAGS                ?= $(BUILDFLAGS_NOPIE) -buildmode=pie
 TESTFLAGS                 ?=
 PWD                       := $(shell pwd)
@@ -25,7 +26,7 @@ export GO111MODULE=on
 
 OK := $(shell tput setaf 6; echo ' [OK]'; tput sgr0;)
 
-all: build completion man
+all: sysinfo build completion man
 build: $(GOPASS_OUTPUT)
 completion: $(BASH_COMPLETION_OUTPUT) $(FISH_COMPLETION_OUTPUT) $(ZSH_COMPLETION_OUTPUT)
 travis: sysinfo crosscompile build fulltest codequality completion full

pkg/clipboard/clipboard.go

@@ -13,8 +13,11 @@ import (
 )
 
 var (
+	// Helpers can be overridden at compile time, e.g. go build \
+	// -ldflags=='-X github.com/gopasspw/gopass/pkg/clipboard.Helpers=termux-api'
+	Helpers = "xsel of xclip"
 	// ErrNotSupported is returned when the clipboard is not accessible
-	ErrNotSupported = fmt.Errorf("WARNING: No clipboard available. Install xsel or xclip or use -f to print to console")
+	ErrNotSupported = fmt.Errorf("WARNING: No clipboard available. Install " + Helpers + " or use -f to print to console")
 )
 
 // CopyTo copies the given data to the clipboard and enqueues automatic