pdf: use printf over echo

echo may introduce inconsistent behaviors over different platforms.  printf is considered better than echo. See: https://unix.stackexchange.com/questions/65803/why-is-printf-better-than-echo

Fixes #188

Author: changkun

pdf/en-us/Makefile

@@ -6,8 +6,8 @@ revision = $(shell git describe --always --tags)
 all: revision pdf
 
 revision:
-	@echo '% Autogenerated, do not edit' > revision.tex
-	@echo '\\newcommand{\\revision}{'$(revision)'}' >> revision.tex
+	printf '%% Autogenerated, do not edit\n' > revision.tex
+	printf '\\newcommand{\\revision}{'$(revision)'}' >> revision.tex
 
 pdf: markdown
 	@echo "Compiling PDF file..."

pdf/zh-cn/Makefile

@@ -6,8 +6,8 @@ revision = $(shell git describe --always --tags)
 all: revision pdf
 
 revision:
-	@echo '% Autogenerated, do not edit' > revision.tex
-	@echo '\\newcommand{\\revision}{'$(revision)'}' >> revision.tex
+	printf '%% Autogenerated, do not edit\n' > revision.tex
+	printf '\\newcommand{\\revision}{'$(revision)'}' >> revision.tex
 
 pdf: markdown
 	@echo "Compiling PDF file..."