add link time optimization and warnings by default

[guijan]

Quoting GCC Documentation:

Link Time Optimization (LTO) gives GCC the capability of dumping its internal representation (GIMPLE) to disk, so that all the different compilation units that make up a single executable can be optimized as a single module. This expands the scope of inter-procedural optimizations to encompass the whole program (or, rather, everything that is visible at link time).

It also has the nice side effect of allowing warnings across translation units, as attested by these internet blogs: gcc 8: link time and interprocedura optimization gnu link time optimization finds non-matching declarations

A new user-visible feature is warning about type mismatches across units.

Some Linux distros are migrating towards enabling it by default for all packages, as Phoronix claims. Some OSes (like OpenBSD) don't touch the compile flags of their packages though, so we should do it ourselves.

This change adds autoconf-archive back as a dependency because I used autoconf-archive macros to get it done. Sorry, I only had the idea after the commit that removed it was pushed to master.

This change replaces the CFLAGS with "-O2 -flto -g" and appends the same flags to LDFLAGS (LTO requires this) if the user hasn't set CFLAGS, effectively changing AC_PROG_CC's default of "-O2 -g". I checked out scrot 1.6, 1.7, and commit 6d299f5 (current master as of writing this), and compiled all 3 of them with CFLAGS='-O2', copying each resulting binary to /tmp/. Then I compiled 6d299f5 with CFLAGS='-O2 -flto' to show the difference made by this change:

$ size /tmp/scrot-*
text    data    bss     dec     hex
45417   2952    392     48761   be79    /tmp/scrot-1.6
43724   3232    208     47164   b83c    /tmp/scrot-1.7
42592   3168    208     45968   b390    /tmp/scrot-6d299f5
39649   2880    200     42729   a6e9    /tmp/scrot-6d299f5-flto

There's an overall trend with scrot getting 5% smaller with all the cleanups that have been made so far, and now LTO on top of 6d299f5 reduces the binary by a further 7% for a total 12% reduction in binary size. LTO and code quality aren't purely there for smaller binaries, but smaller executables are an easy way to notice them.

I also turned on all warnings by default.

[daltomi]

I like it, I've been using it for years.