without this fix (assuming it does, what the original author intended to check for), the configure script spits out an error:

checking for ndk-build                                                                              no
checking for gradle                                                                                 no
${_source}/configure: line 13806: 0: command not found
checking if zsp-gcc supports C99 without any flags                                                  yes
checking if zsp-gcc supports C99 with the -std=c99 flag                                             yes

that's because the condition in AM_CONDITIONAL is expected to be a valid shell expression for an if-statement, not just plain value:

{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking for gradle" >&5
printf %s "checking for gradle... " >&6; }
if test -e ${GRADLE} ; then
    { printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: ${GRADLE}" >&5
printf "%s\n" "${GRADLE}" >&6; }
     if 1; then
  HAVE_GRADLE_TRUE=
  HAVE_GRADLE_FALSE='#'
else
  HAVE_GRADLE_TRUE='#'
  HAVE_GRADLE_FALSE=
fi

else
    { printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: no" >&5
printf "%s\n" "no" >&6; }
     if 0; then
  HAVE_GRADLE_TRUE=
  HAVE_GRADLE_FALSE='#'
else
  HAVE_GRADLE_TRUE='#'
  HAVE_GRADLE_FALSE=
fi

fi

See discussion in #933.

See discussion in #933.

FreeBSD requires __BSD_VISIBLE to be set to expose M_PI and mkstemp() in the
system includes. This is normally set when _ANSI_SOURCE, _C99_SOURCE, or
_C11_SOURCE are undefined. I don't want to dig into what the compiler is
setting, so I'm just setting it in the configure script.

/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-labels    -g -O2 -std=c99 -MT src/libespeak-ng/src_libespeak_ng_la-wavegen.lo -MD -MP -MF src/libespeak-ng/.deps/src_libespeak_ng_la-wavegen.Tpo -c -o src/libespeak-ng/src_libespeak_ng_la-wavegen.lo `test -f 'src/libespeak-ng/wavegen.c' || echo './'`src/libespeak-ng/wavegen.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_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-labels -g -O2 -std=c99 -MT src/libespeak-ng/src_libespeak_ng_la-wavegen.lo -MD -MP -MF src/libespeak-ng/.deps/src_libespeak_ng_la-wavegen.Tpo -c src/libespeak-ng/wavegen.c  -fPIC -DPIC -o src/libespeak-ng/.libs/src_libespeak_ng_la-wavegen.o
src/libespeak-ng/wavegen.c:355:24: error: use of undeclared identifier 'M_PI'
                        x = 127*(1.0 - cos((M_PI*2)*ix/wavemult_max));
                                            ^
src/libespeak-ng/wavegen.c:623:16: error: use of undeclared identifier 'M_PI'
        minus_pi_t = -M_PI / samplerate;
                      ^
2 errors generated.
gmake[1]: *** [Makefile:1690: src/libespeak-ng/src_libespeak_ng_la-wavegen.lo] Error 1