enable coretext by default on Mac

use --disable-coretext explicitely to turn it off (and revert to ATSUI)

Change-Id: I86685482a08cd62665a94eb6e241b59a217d0b52
diff --git a/configure.ac b/configure.ac
index 05a44fb..337af8b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -10663,7 +10663,7 @@ dnl ===================================================================
ENABLE_CORETEXT=NO
if test "$_os" = "Darwin"; then
    AC_MSG_CHECKING([whether to use CoreText framework])
    if test "$enable_coretext" = yes -o "$BITNESS_OVERRIDE" = 64; then
    if test "$enable_coretext" != no -o "$BITNESS_OVERRIDE" = 64; then
        AC_MSG_RESULT([yes])
        ENABLE_CORETEXT=YES
    else
diff --git a/vcl/Library_vcl.mk b/vcl/Library_vcl.mk
index a4a6957..dfaecc7 100644
--- a/vcl/Library_vcl.mk
+++ b/vcl/Library_vcl.mk
@@ -381,10 +381,16 @@ $(eval $(call gb_Library_use_system_darwin_frameworks,vcl,\
	ApplicationServices \
))
else
ifeq ($(MACOSX_SDK_VERSION),1060)
$(eval $(call gb_Library_use_system_darwin_frameworks,vcl,\
	ApplicationServices \
))
else
$(eval $(call gb_Library_use_system_darwin_frameworks,vcl,\
	CoreText \
))
endif
endif

else # ATSUI