Build with optimizations by default (#543)

GCC builds without optimizations by default while Clang uses -O2.
Use -O2 for GCC by default too, but switch to -O0 when DEBUG is
defined. Also make sure to always output debug information, not
only in debug builds. Additional sections with it do not impact
performance but are very useful in production. And they can be
easily stripped out of the binaries if necessary, like we do when
building packages.

We disable frame pointer omission optimization to get better stack
frames and enable debugging on x86. While it has some impact, I'd
rather have working backtraces.

Makefile

@@ -236,13 +236,15 @@ ifdef IS_LINUX
 LDFLAGS += -Wl,--no-undefined
 endif
 
+CFLAGS += -O2 -fno-omit-frame-pointer -g
+
 ifdef COVERAGE
-	CFLAGS += -g -O0 --coverage
+	CFLAGS += -O0 --coverage
 	LDFLAGS += --coverage
 endif
 
 ifdef DEBUG
-	CFLAGS += -DDEBUG -g
+	CFLAGS += -O0 -DDEBUG
 endif
 
 ifneq ($(GEM_INSTALL_OPTIONS),)