The definitions for towlower() and towupper() are defined in /usr/include/wctype.h on FreeBSD platforms. I just removed the define and used  the ucd_ equivilent where towlower/towupper were found.

/bin/sh ./libtool  --tag=CC   --mode=compile cc -DHAVE_CONFIG_H -I.    -fPIC -fvisibility=hidden -pedantic -fno-exceptions -D PATH_ESPEAK_DATA=\"/usr/local/share/espeak-
ng-data\" -DLIBESPEAK_NG_EXPORT  -Isrc/include -Isrc/include/compat -I src/ucd-tools/src/include -D_BSD_SOURCE -D_DEFAULT_SOURCE -D_POSIX_C_SOURCE=200112L -Wno-endif-lab
els    -g -O2 -std=c99 -MT src/libespeak-ng/src_libespeak_ng_la-readclause.lo -MD -MP -MF src/libespeak-ng/.deps/src_libespeak_ng_la-readclause.Tpo -c -o src/libespeak-n
g/src_libespeak_ng_la-readclause.lo `test -f 'src/libespeak-ng/readclause.c' || echo './'`src/libespeak-ng/readclause.c
libtool: compile:  cc -DHAVE_CONFIG_H -I. -fPIC -fvisibility=hidden -pedantic -fno-exceptions -D PATH_ESPEAK_DATA=\"/usr/local/share/espeak-ng-data\" -DLIBESPEAK_NG_EXPO
RT -Isrc/include -Isrc/include/compat -I src/ucd-tools/src/include -D_BSD_SOURCE -D_DEFAULT_SOURCE -D_POSIX_C_SOURCE=200112L -Wno-endif-labels -g -O2 -std=c99 -MT src/li
bespeak-ng/src_libespeak_ng_la-readclause.lo -MD -MP -MF src/libespeak-ng/.deps/src_libespeak_ng_la-readclause.Tpo -c src/libespeak-ng/readclause.c  -fPIC -DPIC -o src/l
ibespeak-ng/.libs/src_libespeak_ng_la-readclause.o
In file included from src/libespeak-ng/readclause.c:32:
In file included from src/include/compat/wctype.h:30:
/usr/include/wctype.h:73:8: error: conflicting types for 'ucd_tolower'
wint_t  towlower(wint_t);
        ^
src/include/compat/wchar.h:37:18: note: expanded from macro 'towlower'
                 ^
src/ucd-tools/src/include/ucd/ucd.h:503:13: note: previous declaration is here
codepoint_t ucd_tolower(codepoint_t c);
            ^
In file included from src/libespeak-ng/readclause.c:32:
In file included from src/include/compat/wctype.h:30:
/usr/include/wctype.h:74:8: error: conflicting types for 'ucd_toupper'
wint_t  towupper(wint_t);
        ^
src/include/compat/wchar.h:38:18: note: expanded from macro 'towupper'
                 ^
src/ucd-tools/src/include/ucd/ucd.h:491:13: note: previous declaration is here
codepoint_t ucd_toupper(codepoint_t c);
            ^
src/libespeak-ng/readclause.c:366:19: warning: implicit declaration of function 'mkstemp' is invalid in C99 [-Wimplicit-function-declaration]
                        if ((fd_temp = mkstemp(fname_temp)) >= 0)
                                       ^
1 warning and 2 errors generated.