Enable and squash compiler warnings

• build: Separate out linker options

  Meson warns that get_supported_arguments() is wrong for linker options like these.

• build: Select C99 language standard at project level

• check-vulkan: Avoid shadowing this->device with function parameter

  Detected by g++ -Wshadow.

• build: Disable -Wdeclaration-after-statement

  It's inconsistent to ask for this in warning_cflags, and immediately disable it in no_warning_cflags.

• build: Split out warning flags that are only supported for C

  For completeness, I've added separate lists of flags that are only supported for C++, although there are none yet.

• build: Disable undesired warnings for C++ too

  In particular this disables -Wmissing-field-initializers in check-vulkan.

• build: Enable extra warning flags for C++ too

• gitlab-ci: Use clang++ for ubsan build

• gitlab-ci: Make compiler warnings into errors

• gitlab-ci: Build ubsan stage with both gcc and clang

  They sometimes give different warnings.

• build: Explicitly disable -Wpedantic

  We're using GLib, and GLib explicitly doesn't support pedantic Standard C compilers: it assumes and requires the ability to cast between function pointers and data pointers.

• build: Disable -Winline

  As noted in pressure-vessel, if the compiler decides not to inline a static inline function (type-safe macro-equivalent), that isn't really a bug.

• build: Use warning level 2 by default

• debian: Enable -Werror for UNRELEASED builds

• Use our own filename for config.h

  This is Meson best-practice: it avoids accidentally including the wrong config.h when using subprojects.

• Define _GNU_SOURCE via -config.h

• build: Consistently use project_include_dirs everywhere

  This matters when invoked as a subproject, for example by pressure-vessel, in which case we don't get -I$(top_builddir).

/cc @denittis @jpwhiting