Add cross-platform buildability (modify preamble.tex automatically)

Use sed to modify preamble.tex to use common fonts on each platform,
so the buid can succeed on any platform.

Theoretically, should be able to build on Windows (+ MinGW, Cygwin,
and any other Windows environment), Mas OS X, Linux, GNU Hurd, *BSD,
Haiku, Android, and any other modern open-source OS.

Makefile

@@ -12,6 +12,20 @@ export TASKS
 
 SHELL=/bin/bash
 
+ifeq ($(OS),Windows_NT) # Windows, MinGW, Cygwin, etc...
+ OS := Windows
+ subst_text := \\setmainfont{Arial}\n\\setmonofont{Courier New}
+else
+ OS := $(shell sh -c 'uname -s 2>/dev/null || echo not')
+ ifeq ($(OS),Darwin) # Mac OS X
+ OS := Darwin
+ subst_text := \\setromanfont{Avenir Next}\n\\setmonofont{Source Code Pro}
+ else # Linux, GNU Hurd, *BSD, Haiku, Android, etc...
+ OS := Linux
+ subst_text := \\setmainfont{Liberation Sans}\n\\setmonofont{Liberation Mono}
+ endif
+endif
+
 docdir = doc
 helpfiles = $(wildcard $(docdir)/*.txt)
 
@@ -26,6 +40,11 @@ update:
 $(docdir):
  ./update.sh
 
+vimhelp.tex vimhelp-a4.tex vimhelp-ipad.tex: preamble.tex
+
+preamble.tex: FORCE
+ sed -i -e '/^\\usepackage{fontspec}/ {n;N;s/.*/$(subst_text)/}' preamble.tex
+
 %.pdf: %.tex body.tex FORCE
  xelatex $<
 ifndef SECOND_TIME_RUN