[vte] parser: Fix warning with GCC < 7

[Christian Persch <chpe src gnome org>]

commit 88239d9fca8365397bd1f069eabf24bc74de95d3
Author: Christian Persch <chpe src gnome org>
Date:   Fri Apr 6 10:01:08 2018 +0200

    parser: Fix warning with GCC < 7
    
    The -Wimplicit-fallthrough option is a GCC 7 addition,
    so don't try ignoring it on GCC < 7.

 src/parser-glue.hh |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/src/parser-glue.hh b/src/parser-glue.hh
index 24b227c..490b06c 100644
--- a/src/parser-glue.hh
+++ b/src/parser-glue.hh
@@ -623,7 +623,9 @@ private:
                 case VTE_SEQ_CSI:
                 case VTE_SEQ_DCS:
                         #pragma GCC diagnostic push
+                        #if G_GNUC_CHECK_VERSION(7, 0)
                         #pragma GCC diagnostic ignored "-Wimplicit-fallthrough"
+                        #endif
                         for (unsigned char n = 0; n < m_n_intermediates; n++)
                                 s.push_back(m_intermediates[n]);
                         /* [[fallthrough]]; */