(cherry picked from commit fda3f64)

(cherry picked from commit c699c93)

This enables the use of different compiler sanitizers, coresponding to
the -fsanitize option in GCC and Clang.

(cherry picked from commit ce0a143)

(cherry picked from commit 76125f6)

Various changes:

 * Don't check $GCC and $GXX
 * Prefer -Og instead of -O0
 * If -g3 isn't available, use -g

This also incidentally fixes compiler warnings with GCC and glibc when using
--enable-debug, as the old default values mixed poorly with the hardening flags.

(cherry picked from commit 45e2031)

(cherry picked from commit 88c9e22)

(cherry picked from commit a8780a9)

Don't optimize at all when --enable-debug is supplied. This makes sure
that nothing is optimized out.

(cherry picked from commit 81ddf86)

Re-adds flags that were removed in 72aa644.

(cherry picked from commit bbdba3b)

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
(cherry picked from commit 6174ba2)

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
(cherry picked from commit 4718c1e)

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
(cherry picked from commit 531e2b2)

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
(cherry picked from commit 9f6278e)

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
(cherry picked from commit 457437e)

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
(cherry picked from commit 6b8c0bc)

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
(cherry picked from commit 6214266)

…d by clean.sh.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
(cherry picked from commit 47b05ab)