Worked around a bug in wxWidgets 3.0.2:
using this version directly adding string constants generated by wxT() results in an error:

  wxString htmldir = wxT(PREFIX) + wxT("/share/doc/wxMaxima");
g++ -DHAVE_CONFIG_H -I.   -I/usr/lib/x86_64-linux-gnu/wx/include/gtk2-unicode-3.0 -I/usr/include/wx-3.0 -D_FILE_OFFSET_BITS=64 -DWXUSINGDLL -D__WXGTK__ -DPREFIX=\"/usr\"  -g -O2  -MT wxMaxima.o -MD -MP -MF .deps/wxMaxima.Tpo -c -o wxMaxima.o wxMaxima.cpp
In file included from /usr/include/wx-3.0/wx/version.h:16:0,
                 from /usr/include/wx-3.0/wx/defs.h:193,
                 from /usr/include/wx-3.0/wx/wx.h:14,
                 from ./wx_inc.h:35,
                 from wxMaximaFrame.h:24,
                 from wxMaxima.h:24,
                 from wxMaxima.cpp:23:
wxMaxima.cpp: In member function ‘void wxMaxima::ShowWxMaximaHelp()’:
/usr/include/wx-3.0/wx/chartype.h:238:44: error: invalid operands of types ‘const wchar_t [5]’ and ‘const wchar_t [20]’ to binary ‘operator+’
             #define wxT(x) wxCONCAT_HELPER(L, x)
                                            ^
/usr/include/wx-3.0/wx/cpp.h:18:37: note: in definition of macro ‘wxCONCAT_HELPER’
 #define wxCONCAT_HELPER(text, line) text ## line
                                     ^
wxMaxima.cpp:1724:36: note: in expansion of macro ‘wxT’
   wxString htmldir = wxT(PREFIX) + wxT("/share/doc/wxMaxima");
                                    ^
wxMaxima.cpp: In member function ‘void wxMaxima::HelpMenu(wxCommandEvent&)’:
wxMaxima.cpp:3633:25: error: ‘IconName’ was not declared in this scope
     info.SetIcon(wxIcon(IconName,wxBITMAP_TYPE_PNG));
                         ^
Makefile:527: recipe for target 'wxMaxima.o' failed
make[2]: *** [wxMaxima.o] Error 1
make[2]: Leaving directory '/home/gunter/src/wxmaxima/src'
Makefile:366: recipe for target 'all' failed
make[1]: *** [all] Error 2