build: add -ffile-prefix-map to CFLAGS

Add -ffile-prefix-map to CFLAGS, which is a way to prevent the full
absolute path of build-time source filenames from ending up in (the
debugging symbols of) the binaries we install.

The RHEL8 version of the code (in /usr/lib/rpm/find-debuginfo.sh) for
scanning for the list of source files to include in the -debugsource
package is confused by the remapping, so disable it.

configure.ac

@@ -247,13 +247,23 @@ AC_DEFINE_UNQUOTED([PATH_SSH_AGENT], ["$SSH_AGENT"], [Location of ssh-agent bina
 
 changequote(,)dnl
 if test "x$GCC" = "xyes"; then
+  if  grep -q platform:el8 /usr/lib/os-release; then
+    # /usr/lib/rpm/find-debuginfo.sh on RHEL 8 gets confused by
+    # -fdebug-prefix-map, so disable it if we're building there
+    strip_sourcefile_prefix="."
+  else
+    strip_sourcefile_prefix="$(realpath -s "${PWD}/${srcdir}")"
+  fi
+
   CFLAGS="-Wall \
           -Werror=strict-prototypes -Werror=missing-prototypes \
           -Werror=implicit-function-declaration \
           -Werror=pointer-arith -Werror=init-self \
           -Werror=format=2 \
           -Werror=return-type \
-          -Werror=missing-include-dirs $CFLAGS"
+          -Werror=missing-include-dirs \
+          -ffile-prefix-map=${strip_sourcefile_prefix}=. \
+          $CFLAGS"
 fi
 changequote([,])dnl