MDEV-3826 compilation of client programs fail: m_string.h tries to include <mysql/plugin.h>

mysql_config:
- add not only $pkgincludedir, but also $pkgincludedir/.. to the header search path,
  for #include <mysql/plugin.h> to work


scripts/mysql_config.sh:
  - don't support headers in */include anymore. only in */include/mysql
  - remove the incorrect "bug fix" (fixed correctly long time ago)
  - add not only $pkgincludedir, but also $pkgincludedir/.. to the header search path,
    for #include <mysql/plugin.h> to work
  - but don't do it, if $pkgincludedir/.. is /usr/include

scripts/mysql_config.sh

@@ -92,11 +92,7 @@ plugindir_rel=`echo $plugindir | sed -e "s;^$basedir/;;"`
 fix_path plugindir $plugindir_rel lib/mysql/plugin lib/plugin
 
 pkgincludedir='@pkgincludedir@'
-if [ -f "$basedir/include/mysql/mysql.h" ]; then
+fix_path pkgincludedir include/mysql
-  pkgincludedir="$basedir/include/mysql"
-elif [ -f "$basedir/include/mysql.h" ]; then
-  pkgincludedir="$basedir/include"
-fi
 
 version='@VERSION@'
 socket='@MYSQL_UNIX_ADDR@'
@@ -125,8 +121,11 @@ if [ -r "$pkglibdir/libmygcc.a" ]; then
   embedded_libs="$embedded_libs -lmygcc "
 fi
 
-cflags="-I$pkgincludedir @CFLAGS@ " #note: end space!
 include="-I$pkgincludedir"
+if [ "$basedir" != "/usr" ]; then
+  include="$include -I$pkgincludedir/.."
+fi
+cflags="$include @CFLAGS@ " #note: end space!
 
 # Remove some options that a client doesn't have to care about
 # FIXME until we have a --cxxflags, we need to remove -Xa