this ends up as -isystem /usr/include, which breaks horribly:

/bin/bash ../../libtool  --tag=CXX   --mode=compile g++ -DPACKAGE_NAME=\"libcmis\" -DPACKAGE_TARNAME=\"libcmis\" -DPACKAGE_VERSION=\"0.6.3\" -DPACKAGE_STRING=\"libcmis\ 0.6.3\" -DPACKAGE_BUGREPORT=\"\" -DPACKAGE_URL=\"\" -DPACKAGE=\"libcmis\" -DVERSION=\"0.6.3\" -DHAVE_STDIO_H=1 -DHAVE_STDLIB_H=1 -DHAVE_STRING_H=1 -DHAVE_INTTYPES_H=1 -DHAVE_STDINT_H=1 -DHAVE_STRINGS_H=1 -DHAVE_SYS_STAT_H=1 -DHAVE_SYS_TYPES_H=1 -DHAVE_UNISTD_H=1 -DSTDC_HEADERS=1 -DHAVE_DLFCN_H=1 -DLT_OBJDIR=\".libs/\" -DHAVE_CXX11=1 -DHAVE_FUNC_ATTRIBUTE_VISIBILITY=1 -DHAVE_BOOST=1 -DHAVE_BOOST_DATE_TIME_POSIX_TIME_POSIX_TIME_HPP=1 -DHAVE_BOOST_DATE_TIME=1 -DHAVE_BOOST_SCOPED_PTR_HPP=1 -DHAVE_BOOST_SHARED_PTR_HPP=1 -DHAVE_BOOST_ALGORITHM_STRING_HPP=1 -DHAVE_BOOST_UUID_UUID_HPP=1 -DHAVE_BOOST_PROGRAM_OPTIONS_HPP=1 -DHAVE_BOOST_PROGRAM_OPTIONS=1 -DHAVE_STDLIB_H=1 -DHAVE_UNISTD_H=1 -DHAVE_STDBOOL_H=1 -I.  -DLIBCMIS_BUILD -DLIBCMIS_VISIBILITY -Wdate-time -D_FORTIFY_SOURCE=2 -I../../inc -I../../src/libcmis -I/usr/include/libxml2 -I/usr/include/x86_64-linux-gnu -isystem /usr/include/mit-krb5 -isystem /usr/include/mit-krb5 -isystem /usr/include -fvisibility=hidden -g -O2 -ffile-prefix-map=/home/rene/Debian/Pakete/libcmis/libcmis=. -fstack-protector-strong -fstack-clash-protection -Wformat -Werror=format-security -fcf-protection -Wall -pedantic -Weffc++ -Wshadow -Wendif-labels -Wextra -Wsign-promo -Woverloaded-virtual -Wnon-virtual-dtor -Wsign-promo -c -o libcmis_la-allowable-actions.lo `test -f 'allowable-actions.cxx' || echo './'`allowable-actions.cxx
libtool: compile:  g++ -DPACKAGE_NAME=\"libcmis\" -DPACKAGE_TARNAME=\"libcmis\" -DPACKAGE_VERSION=\"0.6.3\" "-DPACKAGE_STRING=\"libcmis 0.6.3\"" -DPACKAGE_BUGREPORT=\"\" -DPACKAGE_URL=\"\" -DPACKAGE=\"libcmis\" -DVERSION=\"0.6.3\" -DHAVE_STDIO_H=1 -DHAVE_STDLIB_H=1 -DHAVE_STRING_H=1 -DHAVE_INTTYPES_H=1 -DHAVE_STDINT_H=1 -DHAVE_STRINGS_H=1 -DHAVE_SYS_STAT_H=1 -DHAVE_SYS_TYPES_H=1 -DHAVE_UNISTD_H=1 -DSTDC_HEADERS=1 -DHAVE_DLFCN_H=1 -DLT_OBJDIR=\".libs/\" -DHAVE_CXX11=1 -DHAVE_FUNC_ATTRIBUTE_VISIBILITY=1 -DHAVE_BOOST=1 -DHAVE_BOOST_DATE_TIME_POSIX_TIME_POSIX_TIME_HPP=1 -DHAVE_BOOST_DATE_TIME=1 -DHAVE_BOOST_SCOPED_PTR_HPP=1 -DHAVE_BOOST_SHARED_PTR_HPP=1 -DHAVE_BOOST_ALGORITHM_STRING_HPP=1 -DHAVE_BOOST_UUID_UUID_HPP=1 -DHAVE_BOOST_PROGRAM_OPTIONS_HPP=1 -DHAVE_BOOST_PROGRAM_OPTIONS=1 -DHAVE_STDLIB_H=1 -DHAVE_UNISTD_H=1 -DHAVE_STDBOOL_H=1 -I. -DLIBCMIS_BUILD -DLIBCMIS_VISIBILITY -Wdate-time -D_FORTIFY_SOURCE=2 -I../../inc -I../../src/libcmis -I/usr/include/libxml2 -I/usr/include/x86_64-linux-gnu -isystem /usr/include/mit-krb5 -isystem /usr/include/mit-krb5 -isystem /usr/include -fvisibility=hidden -g -O2 -ffile-prefix-map=/home/rene/Debian/Pakete/libcmis/libcmis=. -fstack-protector-strong -fstack-clash-protection -Wformat -Werror=format-security -fcf-protection -Wall -pedantic -Weffc++ -Wshadow -Wendif-labels -Wextra -Wsign-promo -Woverloaded-virtual -Wnon-virtual-dtor -Wsign-promo -c allowable-actions.cxx  -fPIC -DPIC -o .libs/libcmis_la-allowable-actions.o
In file included from /usr/include/c++/15/ext/string_conversions.h:45,
                 from /usr/include/c++/15/bits/basic_string.h:4442,
                 from /usr/include/c++/15/string:56,
                 from ../../inc/libcmis/allowable-actions.hxx:32,
                 from allowable-actions.cxx:29:
/usr/include/c++/15/cstdlib:83:15: fatal error: stdlib.h: No such file or directory
   83 | #include_next <stdlib.h>
      |               ^~~~~~~~~~
compilation terminated.
make[4]: *** [Makefile:802: libcmis_la-allowable-actions.lo] Error 1
make[4]: Leaving directory '/home/rene/Debian/Pakete/libcmis/libcmis/src/libcmis'
make[3]: *** [Makefile:526: all-recursive] Error 1
make[3]: Leaving directory '/home/rene/Debian/Pakete/libcmis/libcmis/src'
make[2]: *** [Makefile:545: all-recursive] Error 1
make[2]: Leaving directory '/home/rene/Debian/Pakete/libcmis/libcmis'
make[1]: *** [debian/rules:43: override_dh_auto_build] Error 2
make[1]: Leaving directory '/home/rene/Debian/Pakete/libcmis/libcmis'
make: *** [debian/rules:23: binary] Error 2
dpkg-buildpackage: error: debian/rules binary subprocess failed with exit status 2
debuild: fatal error at line 1185:
dpkg-buildpackage -us -uc -ui -b failed

diff --git a/configure.ac b/configure.ac
index 68d2dcb..569a0d2 100644
--- a/configure.ac
+++ b/configure.ac
@@ -170,10 +170,6 @@ BOOST_UUID
 AS_IF([test "x$enable_client" != "xno"], [
 	BOOST_PROGRAM_OPTIONS
 ])
-# Treat boost headers as system headers so -Wshadow / -Weffc++ etc.
-# inside them do not break the build under -Werror. boost.m4 emits
-# -I$prefix; rewrite to -isystem $prefix.
-BOOST_CPPFLAGS=`echo "$BOOST_CPPFLAGS" | sed 's|-I|-isystem |g'`
 AC_SUBST(BOOST_CPPFLAGS)
 
 # Checks for header files.