All of the compile errors in your build log are of the form:
error: narrowing conversion of ‘ddd’ from ‘int’ to ‘char’ inside { } [-Wnarrowing]
There are a great many other compiler diagnostics but they are all warnings - not
errors - that you can live with.
The errors arise from the fact that wxGTK2-2.8.12 - released March, 2011 -
was written to be compiled to an earlier C++ standard (C++98, i.e. C++ 1998)
than the standard that is the default for your g++ compiler, now in July 2017.
Your compiler I assume is g++ 6 or later, which defaults to C++14 (C++ 2014).
Since the C++11 standard, the narrowing conversion that is breaking your build
has been ruled ill-formed, which previously it was not.
You can direct g++ to compile according to the C++ standard of your choice by
passing it the option -std={c++98|c++03|c++11|c++14|c++17}, and you can
pass your choice to the wxGTK2-2.8.12 configure script by including it in
the value of the CXXFLAGS parameter for ./configure. Either of -std=c++98
or -std=c++03 will remove the narrowing conversion errors, e.g.
../configure CXXFLAGS=-std=c++03 --prefix=/opt/wx/2.8 --enable-xrc --enable-monolithic --enable-unicode
Alternatively you might simply direct g++ to suppress the diagnostics denoted by
-Wnarrowing:
../configure CXXFLAGS=-Wno-narrowing --prefix=/opt/wx/2.8 --enable-xrc --enable-monolithic --enable-unicode
The ./configure script of any GNU autotools package (such as you are trying to
build) will have parameters including:
CC C compiler command
CFLAGS C compiler flags
LDFLAGS linker flags, e.g. -L<lib dir> if you have libraries in a
nonstandard directory <lib dir>
CPPFLAGS C/C++ preprocessor flags, e.g. -I<include dir> if you have
headers in a nonstandard directory <include dir>
CPP C preprocessor
CXX C++ compiler command
CXXFLAGS C++ compiler flags
that are there to help you correct for deviations between your toolchain and
the defaults that were expected by the package maintainers when they released the
package. See ./configure --help.