"make pdf" fails on macOS due to incompatibility of echo

[Jun. T <tak...@kb...>]

With the current git master, 'make pdf' fails on two of my Macs.
The problem is in docs/Makefile.am:
                                                                    
201       ( echo "\usepackage{graphicx}" > gpinsetfigure.tex ; \
202         echo "\usepackage{picins}" >> gpinsetfigure.tex ; \        
203         echo "\newcommand{\gpinsetfigure}[1]{" >> gpinsetfigure.tex ; \
204         echo "  \parpic[r][rt]{\includegraphics[width=3in,keepaspectratio]{#1}}" >> gpinsetfigure.tex ; \                                    
205         echo "}" >> gpinsetfigure.tex ; \                        
206       ) ; \                                              
207     $(AM_V_GEN)touch $@                                          
208     $(AM_V_GEN)touch figurestyle

[1] On macOS (at least on some versions), echo builtin of /bin/sh interprets
 '\n' as a newline, and the line 203 will output:

(blank line)
ewcommand{\gpinsetfigure}[1]{

to gpinsetfigure.tex, and pdflatex fails.
(I don't know whether this is a bug or feature of macOS)

[2] (also on Linux, not serious) the '\' at the end of line 206
should be removed. Otherwise, 'make V=0 pdf' gives an error:

    @echo "  GEN     " pdf_figures;touch pdf_figures
/bin/sh: @echo: command not found

A possible patch is attached.

printf builtin is more portable than echo.

One of $(AM_V_GEN) is replaced by $(AM_V_at) because they give the
same output when 'make V=0'.