Fix env var MACOSX_DEPLOYMENT_TARGET to work properly in configure script

Also update Travis to build Xcode 11, and remove Xcode 8 to keep number
of targets limited.

.travis.yml

@@ -6,9 +6,9 @@ os:
 
 osx_image:
   - xcode7.3
-  - xcode8.3
   - xcode9.4
-  - xcode10.2
+  - xcode10.3
+  - xcode11.2
 
 compiler:
   - clang
@@ -23,7 +23,6 @@ addons:
 
 env:
   - MACOSX_DEPLOYMENT_TARGET=10.8
-    XCODEFLAGS="MACOSX_DEPLOYMENT_TARGET=10.8"
     VERSIONER_PERL_VERSION=5.18
     VERSIONER_PYTHON_VERSION=2.7
     vi_cv_path_python=/usr/bin/python

src/auto/configure

@@ -4708,6 +4708,8 @@ rm -f core conftest.err conftest.$ac_objext \
     if test -z "$MACOSX_DEPLOYMENT_TARGET"; then
       macosx_deployment_target=`/usr/bin/sw_vers -productVersion|/usr/bin/sed -e 's/^\([0-9]*\.[0-9]*\).*/\1/'`
       XCODEFLAGS="$XCODEFLAGS MACOSX_DEPLOYMENT_TARGET=$macosx_deployment_target"
+    else
+      XCODEFLAGS="$XCODEFLAGS MACOSX_DEPLOYMENT_TARGET=$MACOSX_DEPLOYMENT_TARGET"
     fi
   fi
 
@@ -9183,7 +9185,7 @@ $as_echo "no GUI support" >&6; }
 		SKIP_MACVIM=YES ;;
     yes|""|auto)	{ $as_echo "$as_me:${as_lineno-$LINENO}: result: yes/auto - automatic GUI support" >&5
 $as_echo "yes/auto - automatic GUI support" >&6; }
-    	gui_auto=yes
+        gui_auto=yes
 		SKIP_CARBON=YES ;;
     carbon)	{ $as_echo "$as_me:${as_lineno-$LINENO}: result: Carbon GUI support" >&5
 $as_echo "Carbon GUI support" >&6; }

src/configure.ac

@@ -236,6 +236,8 @@ if test "`(uname) 2>/dev/null`" = Darwin; then
     if test -z "$MACOSX_DEPLOYMENT_TARGET"; then
       macosx_deployment_target=`/usr/bin/sw_vers -productVersion|/usr/bin/sed -e 's/^\([[0-9]]*\.[[0-9]]*\).*/\1/'`
       XCODEFLAGS="$XCODEFLAGS MACOSX_DEPLOYMENT_TARGET=$macosx_deployment_target"
+    else
+      XCODEFLAGS="$XCODEFLAGS MACOSX_DEPLOYMENT_TARGET=$MACOSX_DEPLOYMENT_TARGET"
     fi
   fi