[LyX/master] Revert "Limit the -Wall flag to C++ compiler"

[Jean-Marc Lasgouttes]

commit ff1ab048f10895c5106469c6b63fe78a290402d8
Author: Jean-Marc Lasgouttes <lasgouttes at lyx.org>
Date:   Fri Nov 20 14:38:06 2020 +0100

    Revert "Limit the -Wall flag to C++ compiler"
    
    Something is fishy, it breaks compilation.
    
    This reverts commit cbc9a901afd1783a30b4d76663208b0c13530a30.
---
 config/lyxinclude.m4 |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/config/lyxinclude.m4 b/config/lyxinclude.m4
index cf58df7..268c501 100644
--- a/config/lyxinclude.m4
+++ b/config/lyxinclude.m4
@@ -418,7 +418,7 @@ if test x$GXX = xyes; then
   fi
   dnl Warnings are for preprocessor too
   if test x$enable_warnings = xyes ; then
-      AM_CXXFLAGS="$AM_CPPFLAGS -Wall -Wextra"
+      AM_CPPFLAGS="$AM_CPPFLAGS -Wall -Wextra"
       dnl Shut off warning -Wdeprecated-copy, which triggers too much
       dnl note that g++ always accepts -Wno-xxx, even when -Wxxx is an error.
       AC_LANG_PUSH(C++)