gnu: espeak: Build with the C++ 98 standard.

Fixes <https://bugs.gnu.org/37672>.
Reported by Bradley Haggerty <bradigger@gmail.com>.

* gnu/packages/speech.scm (espeak)[arguments]: Add CXXFLAGS to #:make-flags.

gnu/packages/speech.scm

@@ -69,6 +69,8 @@ (define-public espeak
                           (string-append "LDFLAGS=-Wl,-rpath="
                                          (assoc-ref %outputs "out")
                                          "/lib")
+                          ;; The package fails to build with newer C++ standards.
+                          "CXXFLAGS=-std=c++98"
                           "AUDIO=pulseaudio")
        #:tests? #f ; no check target
        #:phases