out with the -O3, in with the -O2

-O3 does not measurably improve performance of the resulting binaries,
neither with lto enabled nor with lto disabled. what it does to however
is cause gcc warning spew in libstdc++ that we can't do anything
about (and that upon inspection of libstdc++ source looks like a gcc
bug).

with lto, -O3:

Benchmark 1: GC_INITIAL_HEAP_SIZE=10g nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
    Time (mean ± σ):      4.608 s ±  0.027 s    [User: 3.866 s, System: 0.522 s]
    Range (min … max):    4.579 s …  4.640 s    10 runs

Benchmark 2:  nix eval -f <nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix>
    Time (mean ± σ):     408.1 ms ±  25.5 ms    [User: 360.0 ms, System: 28.1 ms]
    Range (min … max):   387.6 ms … 439.0 ms    10 runs

with lto, -O2:

Benchmark 1: GC_INITIAL_HEAP_SIZE=10g nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
    Time (mean ± σ):      4.632 s ±  0.044 s    [User: 3.874 s, System: 0.544 s]
    Range (min … max):    4.563 s …  4.673 s    10 runs

Benchmark 2:  nix eval -f <nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix>
    Time (mean ± σ):     394.0 ms ±  23.9 ms    [User: 351.2 ms, System: 27.6 ms]
    Range (min … max):   377.8 ms … 429.3 ms    10 runs

without lto, -O3:

Benchmark 1: GC_INITIAL_HEAP_SIZE=10g nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
    Time (mean ± σ):      4.700 s ±  0.024 s    [User: 3.906 s, System: 0.559 s]
    Range (min … max):    4.663 s …  4.717 s    10 runs

Benchmark 2:  nix eval -f <nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix>
    Time (mean ± σ):     400.4 ms ±  25.6 ms    [User: 353.7 ms, System: 26.8 ms]
    Range (min … max):   379.8 ms … 430.6 ms    10 runs

without lto, -O2:

Benchmark 1: GC_INITIAL_HEAP_SIZE=10g nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
    Time (mean ± σ):      4.724 s ±  0.030 s    [User: 3.924 s, System: 0.570 s]
    Range (min … max):    4.687 s …  4.749 s    10 runs

Benchmark 2:  nix eval -f <nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix>
    Time (mean ± σ):     392.4 ms ±  24.3 ms    [User: 350.9 ms, System: 26.4 ms]
    Range (min … max):   376.9 ms … 428.0 ms    10 runs

fixes #46

Change-Id: Ib8afad8a07c278f57f2e3317d00cce4f9ec0f338

Makefile

@@ -61,7 +61,7 @@ endif
 OPTIMIZE = 1
 
 ifeq ($(OPTIMIZE), 1)
-  GLOBAL_CXXFLAGS += -O3 $(CXXLTO)
+  GLOBAL_CXXFLAGS += -O2 $(CXXLTO)
   GLOBAL_LDFLAGS += $(CXXLTO)
 else
   GLOBAL_CXXFLAGS += -O0 -U_FORTIFY_SOURCE