Indent C preprocessor directives to reflect their nesting

Current versions

cppi requires the following formula to be installed:
xz 5.2.3 General-purpose data compression with high compression ratio

Recent formula history

ilovezfs Use “squiggly” heredocs.
ilovezfs cppi: swap GNU url and mirror.
ilovezfs cppi: fix mirror (#11672)
Dominyk Tiller urls: update ftpmirror to use https
Nikolaus Wittenstein Add descriptions to all remaining homebrew packages

Formula code at GitHub