patch 8.2.0541: Travis CI does not give compiler warnings

Problem:    Travis CI does not give compiler warnings.
Solution:   Add flags for warnings.  Fix uncovered problems. (Ozaki Kiichi,
            closes #5898)

ci/config.mk.clang.sed

@@ -0,0 +1 @@
+/^RUBY_CFLAGS\b/s/$/ -Wno-error=unknown-attributes -Wno-error=ignored-attributes/

ci/config.mk.gcc.sed

@@ -0,0 +1 @@
+/^CFLAGS\b/s/$/ -Wno-error=maybe-uninitialized/

ci/config.mk.sed

@@ -0,0 +1,2 @@
+/^CFLAGS\b/s/$/ -Wall -Wextra -Wshadow -Werror/
+/^PERL_CFLAGS\b/s/$/ -Wno-error=unused-function/