[glibmm] manual refactor target: Error if clang-format is not available.

[Murray Cumming <murrayc src gnome org>]

commit da03715eb0bba919e82e01e041d2afc842807e99
Author: Murray Cumming <murrayc murrayc com>
Date:   Sat Feb 27 15:28:06 2016 +0100

    manual refactor target: Error if clang-format is not available.

 Makefile.am  |    4 ++++
 configure.ac |    3 ++-
 2 files changed, 6 insertions(+), 1 deletions(-)
---
diff --git a/Makefile.am b/Makefile.am
index 7473185..a0df8af 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -57,5 +57,9 @@ DISTCLEANFILES = $(filter %mmconfig.h,$(msvc_files))
 include $(top_srcdir)/build/dist-changelog.am
 
 reformat:
+if HAVE_CLANG_FORMAT
        $(CLANG_FORMAT) -i `find . -name "*.h"`
        $(CLANG_FORMAT) -i `find . -name "*.cc"`
+else
+       $(error clang-format was not not found during configure.)
+endif
diff --git a/configure.ac b/configure.ac
index b7f88f8..3832689 100644
--- a/configure.ac
+++ b/configure.ac
@@ -129,7 +129,8 @@ AC_DEFINE([GLIBMM_PROPERTIES_ENABLED],[1], [This is always set. This is only for
 AC_DEFINE([GLIBMM_VFUNCS_ENABLED],[1], [This is always set. This is only for backwards compatibility.])
 AC_DEFINE([GLIBMM_DEFAULT_SIGNAL_HANDLERS_ENABLED],[1], [This is always set. This is only for backwards 
compatibility.])
 
-AC_CHECK_PROGS([CLANG_FORMAT], [clang-format clang-format-3.9 clang-format-3.8 clang-format-3.7], 
[clang-format])
+AC_CHECK_PROGS([CLANG_FORMAT], [clang-format clang-format-3.9 clang-format-3.8 clang-format-3.7], [])
+AM_CONDITIONAL([HAVE_CLANG_FORMAT], test -n "$CLANG_FORMAT")
 
 AC_CONFIG_FILES([Makefile
                  tools/Makefile