code cleanup: header detection with has_include

src/include/compat/endian.h

 #define ENDIAN_H_COMPAT_SHIM
 #pragma once
 
-#if defined(HAVE_ENDIAN_H)
+#if __has_include_next(<endian.h>)
 #	pragma GCC system_header // Silence "warning: #include_next is a GCC extension"
 #	include_next <endian.h>
-#elif defined(HAVE_SYS_ENDIAN_H)
+#elif __has_include(<sys/endian.h>)
 #	include <sys/endian.h>
 #	if !defined(be16toh)
 #		define be16toh(x) betoh16(x)

src/include/compat/getopt.h

 #ifndef GETOPT_H_COMPAT_SHIM
 #define GETOPT_H_COMPAT_SHIM
 
-#if defined(HAVE_GETOPT_H)
+#if __has_include_next(<getopt.h>)
 #pragma GCC system_header // Silence "warning: #include_next is a GCC extension"
 #include_next <getopt.h>
 #else

src/include/compat/stdint.h

 
 #pragma GCC system_header // Silence "warning: #include_next is a GCC extension"
 
-#if defined(HAVE_STDINT_H) || !defined(HAVE_INTTYPES_H)
+#if __has_include(<stdint.h>) || !__has_include(<inttypes.h>)
 #include_next <stdint.h>
 #else
 #include_next <inttypes.h>

src/include/compat/unistd.h

 #ifndef UNISTD_H_COMPAT_SHIM
 #define UNISTD_H_COMPAT_SHIM
 
-#if defined(HAVE_UNISTD_H)
+#if __has_include_next(<unistd.h>)
 #pragma GCC system_header // Silence "warning: #include_next is a GCC extension"
 #include_next <unistd.h>
 #endif

src/libespeak-ng/readclause.c

 
 #include "readclause.h"
 #include "common.h"               // for GetFileLength, strncpy0
-#include "config.h"               // for HAVE_MKSTEMP
 #include "dictionary.h"           // for LookupDictList, DecodePhonemes, Set...
 #include "error.h"                // for create_file_error_context
 #include "phoneme.h"              // for phonSWITCH

src/libespeak-ng/speech.h

 #endif
 
 #ifndef MAKE_MEM_UNDEFINED
-#  ifdef HAVE_VALGRIND_MEMCHECK_H
+#  if __has_include(<valgrind/memcheck.h>)
 #    include <valgrind/memcheck.h>
 #    define MAKE_MEM_UNDEFINED(addr, len) VALGRIND_MAKE_MEM_UNDEFINED(addr, len)
 #  else