Building fontconfig in a sid chroot with:
dpkg-buildpackage --no-sign --build=binary

leads to:
...
libtool: compile:  gcc -DHAVE_CONFIG_H -I. -I.. -I.. -I../src
-I/usr/include/freetype2 -Wall -Wpointer-arith -Wstrict-prototypes
-Wmissing-prototypes -Wmissing-declarations -Wnested-externs
-fno-strict-aliasing -DFC_CACHEDIR=\"/var/cache/fontconfig\"
-DFONTCONFIG_PATH=\"/etc/fonts\" -Wdate-time -D_FORTIFY_SOURCE=2 -g -O2
-fdebug-prefix-map=/home/actionmystique/src/Fontconfig/fontconfig-2.12.3-0.2=.
-fstack-protector-strong -Wformat -Werror=format-security -pthread -c
fcobjs.c  -fPIC -DPIC -o .libs/fcobjs.o
In file included from fcobjs.c:33:0:
fcobjshash.gperf:28:1: error: conflicting types for 'FcObjectTypeHash'
 "aspect",FC_ASPECT_OBJECT
 ^~~~~~~~~~~~~~~~
fcobjs.c:28:1: note: previous declaration of 'FcObjectTypeHash' was here
 FcObjectTypeHash (register const char *str, register FC_GPERF_SIZE_T len);
 ^~~~~~~~~~~~~~~~
In file included from fcobjs.c:33:0:
fcobjshash.gperf:172:1: error: conflicting types for 'FcObjectTypeLookup'
fcobjs.c:31:1: note: previous declaration of 'FcObjectTypeLookup' was here
 FcObjectTypeLookup (register const char *str, register FC_GPERF_SIZE_T
len);
 ^~~~~~~~~~~~~~~~~~
Makefile:638: recipe for target 'fcobjs.lo' failed
make[4]: *** [fcobjs.lo] Error 1
make[4]: Leaving directory
'/home/actionmystique/src/Fontconfig/fontconfig-2.12.3-0.2/src'
Makefile:499: recipe for target 'all' failed
make[3]: *** [all] Error 2
make[3]: Leaving directory
'/home/actionmystique/src/Fontconfig/fontconfig-2.12.3-0.2/src'
Makefile:582: recipe for target 'all-recursive' failed
make[2]: *** [all-recursive] Error 1
make[2]: Leaving directory
'/home/actionmystique/src/Fontconfig/fontconfig-2.12.3-0.2'
Makefile:466: recipe for target 'all' failed
make[1]: *** [all] Error 2
make[1]: Leaving directory
'/home/actionmystique/src/Fontconfig/fontconfig-2.12.3-0.2'
/usr/share/cdbs/1/class/makefile.mk:77: recipe for target
'debian/stamp-makefile-build' failed
make: *** [debian/stamp-makefile-build] Error 2
dpkg-buildpackage: error: debian/rules build subprocess returned exit
status 2

Whole build log is attached.