Enable and fix additional build warnings #827

Job	Run time
manyconfig	24m 19s
Windows (Win32)	2m 8s
GCC -O0	1m 55s
GCC -Os, old Autotools	1m 0s
Clang (-O0)	2m 28s
GCC -O3	55s
Windows (x64)	2m 8s
Clang (-O2)	3m 39s
GCC -Os, CMake+ninja, no JIT	1m 3s
Bazel	0s
perl	20s
CMake unity build	39s
Check autogenerated file freshness	18s
Zig	0s
	40m 52s

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