Remove excessive quoting from configure

Don't quote AC_MSG_XXX() macros, this is unnecessary.

configure.ac

@@ -43,7 +43,7 @@ AC_ARG_VAR(WX_CONFIG, [Path to wx-config script (default: search in PATH).])
 WX_CONFIG_OPTIONS
 WX_CONFIG_CHECK([2.8.0],
     [],
-    [AC_MSG_FAILURE([wxWidgets required but not detected.])],
+    AC_MSG_FAILURE([wxWidgets required but not detected.]),
     [base,core,xml]
 )
 
@@ -55,7 +55,7 @@ dnl yet).
 core_WX_LIBS=$WX_LIBS
 WX_CONFIG_CHECK([2.8.0],
     [WX_LIBS_PDFDC_SAMPLE=$WX_LIBS],
-    [AC_MSG_WARN([Some wxWidgets libraries not available, pdfdc sample won't be built])],
+    AC_MSG_WARN([Some wxWidgets libraries not available, pdfdc sample won't be built]),
     [adv,base,core,html,richtext,xml]
 )
 WX_LIBS=$core_WX_LIBS
@@ -96,7 +96,7 @@ if test -n "$USE_GTK"; then
     PKG_CHECK_MODULES(FONTCONFIG, fontconfig,
                       [CXXFLAGS="$CXXFLAGS $FONTCONFIG_CFLAGS"
                        LIBS="$LIBS $FONTCONFIG_LIBS"],
-                      [AC_MSG_FAILURE(Required Fontconfig library not detected.)])
+                      AC_MSG_FAILURE(Required Fontconfig library not detected.))
 fi
 
 dnl This is needed by WX_LIKE_LIBNAME