[gimp] configure.ac: require a C++14 compiler

[N/A <ell src gnome org>]

commit 76eedf21987c8bb5df1e9f6e1dff69a08b9b0505
Author: Ell <ell_se yahoo com>
Date:   Thu Apr 19 04:11:07 2018 -0400

    configure.ac: require a C++14 compiler
    
    This didn't take long :)  Require a C++14 compiler, for polymorphic
    lambdas, which are used in the next commit.

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 9ab8074..32f7bd7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -237,7 +237,7 @@ AM_PROG_CC_C_O
 
 # Determine a C++ compiler to use
 AC_PROG_CXX
-AX_CXX_COMPILE_STDCXX([11], , [mandatory])
+AX_CXX_COMPILE_STDCXX([14], , [mandatory])
 
 # Initialize libtool
 LT_PREREQ([2.2])