configure: Fix and simplify GTK3 detection

Quote the package name "gtk+-3.0" when passing it to the
PKG_CHECK_MODULES() to avoid issues. Also simplify setting BUILD_GUI
using AM_CONDITIONAL().

Signed-off-by: Damien Le Moal <[email protected]>

Author: damien-lemoal

configure.ac

@@ -73,10 +73,9 @@ AC_ARG_ENABLE([gui],
 			[Disable build of GUI tools (gzbc and gzviewer) [default=no]]))
 AS_IF([test "x$enable_gui" != "xno"],
       [m4_ifdef([PKG_CHECK_MODULES],
-		[PKG_CHECK_MODULES([GTK], [gtk+-3.0], [HAVE_GTK3=1], [HAVE_GTK3=0])],
-		[HAVE_GTK3=0])
-       AM_CONDITIONAL([BUILD_GUI], [test "$HAVE_GTK3" -eq 1])],
-      [AM_CONDITIONAL([BUILD_GUI], false)])
+		[PKG_CHECK_MODULES([GTK], ["gtk+-3.0"], [HAVE_GTK=1], [HAVE_GTK=0])],
+		[HAVE_GTK=0])])
+AM_CONDITIONAL([BUILD_GUI], [test "$HAVE_GTK" -eq 1])
 
 # Build test suite
 AC_ARG_WITH([test],