build: Remove all optimization toggles from CFLAGS

Some build environments inject optimization flags in the CFLAGS
variable, but we really don't want that to happen for Graphene,
since we're setting -O3 ourselves in our base CFLAGS.
alloc: Quell a compiler warning in a fallback path

The err variable should be set in all conditional branches.
bench: Ensure the right compiler/linker flags are used

Since we're making the benchmark test suite access the SIMD instructions
directly, we need to specify the compiler and linker flags in the build;
only using GLib's compiler and linker flags won't help.

[ebassi/graphene#13]