Fixes compilation error on std=gnu++14

[erikwijmans]

Motivation and Context

On std=gnu++14, I get the following compile error:

[30/10/44  11% :: 8.5] Building CXX object src/Corrade/PluginManager/CMakeFiles/CorradePluginManagerObjects.dir/AbstractPlugin.cpp.o
FAILED: src/Corrade/PluginManager/CMakeFiles/CorradePluginManagerObjects.dir/AbstractPlugin.cpp.o
/Library/Developer/CommandLineTools/usr/bin/c++  -DCorradePluginManagerObjects_EXPORTS -I../src -Isrc -std=gnu++14 -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk -fPIC   -Wall -Wextra -Wold-style-cast -Winit-self -Werror=return-type -Wmissing-declarations -pedantic -fvisibility=hidden -Wmissing-prototypes -Wno-shorten-64-to-32 -MD -MT src/Corrade/PluginManager/CMakeFiles/CorradePluginManagerObjects.dir/AbstractPlugin.cpp.o -MF src/Corrade/PluginManager/CMakeFiles/CorradePluginManagerObjects.dir/AbstractPlugin.cpp.o.d -o src/Corrade/PluginManager/CMakeFiles/CorradePluginManagerObjects.dir/AbstractPlugin.cpp.o -c ../src/Corrade/PluginManager/AbstractPlugin.cpp
In file included from ../src/Corrade/PluginManager/AbstractPlugin.cpp:26:
In file included from ../src/Corrade/PluginManager/AbstractPlugin.h:32:
../src/Corrade/Containers/Pointer.h:118:123: error: chosen constructor is explicit in copy-initialization
        template<class ...Args> explicit Pointer(InPlaceInitT, Args&&... args): _pointer{new T{std::forward<Args>(args)...}} {}
                                                                                                                          ^
../src/Corrade/PluginManager/AbstractPlugin.cpp:51:35: note: in instantiation of function template specialization 'Corrade::Containers::Pointer<Corrade::PluginManager::AbstractPlugin::State>::Pointer<>' requested here
AbstractPlugin::AbstractPlugin(): _state{Containers::InPlaceInit} {}
                                  ^
../src/Corrade/Utility/ConfigurationGroup.h:59:18: note: explicit constructor declared here
        explicit ConfigurationGroup();
                 ^
../src/Corrade/PluginManager/AbstractPlugin.cpp:38:33: note: in implicit initialization of field 'configuration' with omitted initializer
    Utility::ConfigurationGroup configuration;
                                ^
1 error generated.
[30/1/44  31% :: 10.3] Building CXX object src/Corrade/TestSuite/CMakeFiles/CorradeTestSuite.dir/Tester.cpp.o
ninja: build stopped: subcommand failed.

This PR explicitly initializes configuration, fixing this error. On this PR, I can successfully build corrade (and all of magnum for that matter) with std=gnu++14

How to repro

mkdir build && cd build
cmake -DCMAKE_CXX_FLAGS="-std=gnu++14" ..
make

[mosra]

Hi, thank you!

One very weird request before I am able to look at this -- there's this very nasty security feature bug in Travis CI where if you set it up for your fork on travis-ci.com while I have it for my repo on travis-ci.org, I will stop getting notifications on my own builds and start getting notifications from your builds instead, like this:

(Also note there are no Travis checks displayed below this PR.) The only solution to this I know is either:

• if you either fully remove the Travis setup for Corrade from your travis-ci.com account
• or you remove it from travis-ci.com and set it up on travis-ci.org instead

This is a very nasty bug, sorry about the inconvenience. I didn't have the nerve to report it to Travis themselves because their issue tracker is the worst.

[codecov-io]

Codecov Report

Merging #62 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
+ Coverage   97.63%   97.63%   +<.01%     
==========================================
  Files          80       80              
  Lines        5233     5234       +1     
==========================================
+ Hits         5109     5110       +1     
  Misses        124      124

Impacted Files	Coverage Δ
src/Corrade/PluginManager/AbstractPlugin.cpp	75.86% <100%> (+0.86%)	⬆️

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[mosra]

Merged as 8270822, thank you! While testing locally, this happened only with Clang, GCC 8.2 with -std=gnu++14 had no problem with the original code.