fix compilation warnings and enable the -Werror flag in CI #251

Job	Run time
ubuntu-latest\|debug\|--enable-mcsat\|CC=gcc CXX=g++	9m 55s
ubuntu-latest\|debug\|--enable-mcsat\|CC=clang CXX=clang++	9m 21s
ubuntu-latest\|debug\|--enable-thread-safety\|CC=gcc CXX=g++	3m 21s
ubuntu-latest\|debug\|--enable-thread-safety\|CC=clang CXX=clang++	4m 27s
ubuntu-latest\|release\|--enable-mcsat\|CC=gcc CXX=g++	6m 11s
ubuntu-latest\|release\|--enable-mcsat\|CC=clang CXX=clang++	5m 48s
ubuntu-latest\|release\|--enable-thread-safety\|CC=gcc CXX=g++	3m 14s
ubuntu-latest\|release\|--enable-thread-safety\|CC=clang CXX=clang++	3m 28s
ubuntu-latest\|gcov\|--enable-mcsat\|CC=gcc CXX=g++	10m 45s
ubuntu-latest\|debug\|--enable-thread-safety --enable-mcsat\|CC=gcc CXX=g++	9m 44s
	1h 6m 14s

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