patch 8.0.0464: can't find executable name on Solaris and FreeBSD

Problem:    Can't find executable name on Solaris and FreeBSD.
Solution:   Check for "/proc/self/path/a.out". (Danek Duvall) And for
            "/proc/curproc/file".
diff --git a/src/configure.ac b/src/configure.ac
index 4445cf5..6251681 100644
--- a/src/configure.ac
+++ b/src/configure.ac
@@ -3020,12 +3020,21 @@
 dnl end of GUI-checking
 dnl ---------------------------------------------------------------------------
 
-AC_MSG_CHECKING([for /proc/self/exe])
+AC_MSG_CHECKING([for /proc link to executable])
 if test -L "/proc/self/exe"; then
-    AC_MSG_RESULT(yes)
-    AC_DEFINE(HAVE_PROC_SELF_EXE)
+  dnl Linux
+  AC_MSG_RESULT([/proc/self/exe])
+  AC_DEFINE(PROC_EXE_LINK, "/proc/self/exe")
+elif test -L "/proc/self/path/a.out"; then
+  dnl Solaris
+  AC_MSG_RESULT([/proc/self/path/a.out])
+  AC_DEFINE(PROC_EXE_LINK, "/proc/self/path/a.out")
+elif test -L "/proc/curproc/file"; then
+  dnl FreeBSD
+  AC_MSG_RESULT([/proc/curproc/file])
+  AC_DEFINE(PROC_EXE_LINK, "/proc/curproc/file")
 else
-    AC_MSG_RESULT(no)
+  AC_MSG_RESULT(no)
 fi
 
 dnl Check for Cygwin, which needs an extra source file if not using X11