build: quiet annoying warnings without adding new ones #7954

addresses #7394 and some of #7902

Disabling warnings can be tricky, because doing so can cause a different compiler to create new warnings about unsupported disable flags. Also, some warnings don't surface until they're paired with another warning (gcc). For example, adding "-Wno-foo" won't cause any trouble, but if there's a legitimate warning emitted, the "unknown option -Wno-foo" will show up as well.

Work around this in 2 ways:

1. When checking to see if -Wno-foo is supported, check for "-Wfoo" instead.
2. Enable -Werror while checking 1.

If "-Werror -Wfoo" compiles, "-Wno-foo" is almost guaranteed to be supported.

-Werror itself is also checked. If that fails to compile by itself, it likely means that the user added a flag that adds a warning. In that case, -Werror won't be used while checking, and the build may be extra noisy. The user would need to fix the bad input flag.

Also, silence 2 more additional warnings that can show up post-c++11.

ACK https://github.com/bitcoin/bitcoin/pull/7954/commits/63b3111f84daa421abc2889a550e86c17bf9609d

Tested ACK 63b3111

ACK 63b3111

Diff of the output of ./autogen.sh + ./configure --enable-hardening with the patch.

@@ -163,6 +163,15 @@ checking for readelf... no
 checking for c++filt... /usr/bin/c++filt
 checking for pkg-config... /usr/local/bin/pkg-config
 checking pkg-config is at least version 0.9.0... yes
-checking whether C++ compiler accepts -Werror... yes
-checking whether C++ compiler accepts -Wall... yes
-checking whether C++ compiler accepts -Wextra... yes
-checking whether C++ compiler accepts -Wformat... yes
-checking whether C++ compiler accepts -Wformat-security... yes
-checking whether C++ compiler accepts -Wunused-parameter... yes
-checking whether C++ compiler accepts -Wself-assign... yes
-checking whether C++ compiler accepts -Wunused-local-typedef... yes
-checking whether C++ compiler accepts -Wdeprecated-register... yes
 checking for port... no
 checking for brew... brew
 checking whether the linker accepts -Wl,-headerpad_max_install_names... yes