mylodon's failures in the back branches

The reason that mylodon has been failing in v10 and v11 for awhile
is that "-Werror=c99-extensions" breaks its test for <stdbool.h>:

configure:12708: checking for stdbool.h that conforms to C99
configure:12775: ccache clang-13 -c -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -Wno-unused-command-line-argument -Wno-compound-token-split-by-macro -g -O1 -ggdb -g3 -fno-omit-frame-pointer -Wall -Wextra -Wno-unused-parameter -Wno-sign-compare -Wno-missing-field-initializers -Wno-array-bounds -std=c89 -Wc99-extensions -Werror=c99-extensions -D_GNU_SOURCE -I/usr/include/libxml2 conftest.c >&5
conftest.c:83:25: error: '_Bool' is a C99 extension [-Werror,-Wc99-extensions]
struct s { _Bool s: 1; _Bool t; } s;
^

which causes us to not use stdbool.h, which might be all right if you
weren't also specifying --with-icu.

What's not quite clear to me is what changed on mylodon to make it
start failing recently. Maybe you updated ICU to a version that
insists on importing <stdbool.h> in its headers?

regards, tom lane