wxWidgets: Fix builds (#6560)

- depends on GTK+ with X enabled
  (btw, non-X backends are deprecated/removed in GTK+)
- patch for 3.0.1-3.0.2: missing include caused multiple
  `error: call of overloaded ‘abs(XYPOSITION)’ is ambiguous`
  (seen in GCC 6.3.0); fixed in 3.0.3+

var/spack/repos/builtin/packages/wx/math_include.patch

@@ -0,0 +1,12 @@
+diff --git a/src/stc/scintilla/src/Editor.cxx b/src/stc/scintilla/src/Editor.cxx
+index cd72953ae7..8c19154313 100644
+--- a/src/stc/scintilla/src/Editor.cxx
++++ b/src/stc/scintilla/src/Editor.cxx
+@@ -10,6 +10,7 @@
+ #include <stdio.h>
+ #include <ctype.h>
+ #include <assert.h>
++#include <math.h>
+
+ #include <string>
+ #include <vector>

var/spack/repos/builtin/packages/wx/package.py

@@ -44,8 +44,10 @@ class Wx(AutotoolsPackage):
 
     version('develop', git='https://github.com/wxWidgets/wxWidgets.git', branch='master')
 
+    patch('math_include.patch', when='@3.0.1:3.0.2')
+
     depends_on('pkgconfig', type='build')
-    depends_on('gtkplus')
+    depends_on('gtkplus+X')
 
     @when('@:3.0.2')
     def build(self, spec, prefix):