doc: don't let Doxygen see Platform::WindowlessApplication.

This name isn't known to it at the time it parses the header (because no
such header gets included for it), which in turn causes Doxygen 1.12 to
generate a dummy ::Platform namespace. Which then gets a priority over
Magnum::Platform when linked to, and because it's dummy, it's reported
as an error because it's not allowed to link to undocumented stuff.

src/Magnum/GL/OpenGLTester.h

@@ -304,10 +304,12 @@ class OpenGLTester: public TestSuite::Tester {
         void gpuTimeBenchmarkBegin();
         std::uint64_t gpuTimeBenchmarkEnd();
 
+        #ifndef DOXYGEN_GENERATING_OUTPUT
         struct WindowlessApplication: Platform::WindowlessApplication {
             explicit WindowlessApplication(const Arguments& arguments): Platform::WindowlessApplication{arguments} {}
             int exec() override final { return 0; }
         } _windowlessApplication;
+        #endif
 
         TimeQuery _gpuTimeQuery{NoCreate};
 };

src/Magnum/Platform/gl-info.cpp

@@ -200,6 +200,7 @@ Vendor extension support:
     for more information.
 */
 
+#ifndef DOXYGEN_GENERATING_OUTPUT
 class MagnumInfo: public Platform::WindowlessApplication {
     public:
         explicit MagnumInfo(const Arguments& arguments);
@@ -873,6 +874,7 @@ MagnumInfo::MagnumInfo(const Arguments& arguments): Platform::WindowlessApplicat
     #undef _l
     #undef _h
 }
+#endif
 
 }
 

src/Magnum/Text/fontconverter.cpp

@@ -125,6 +125,7 @@ The resulting font files can be then used as specified in the documentation of
 
 namespace Text {
 
+#ifndef DOXYGEN_GENERATING_OUTPUT
 class FontConverter: public Platform::WindowlessApplication {
     public:
         explicit FontConverter(const Arguments& arguments);
@@ -228,6 +229,7 @@ int FontConverter::exec() {
 
     return 0;
 }
+#endif
 
 }
 

src/Magnum/TextureTools/distancefieldconverter.cpp

@@ -131,6 +131,7 @@ multiple of 2.
 
 namespace TextureTools {
 
+#ifndef DOXYGEN_GENERATING_OUTPUT
 class DistanceFieldConverter: public Platform::WindowlessApplication {
     public:
         explicit DistanceFieldConverter(const Arguments& arguments);
@@ -250,6 +251,7 @@ int DistanceFieldConverter::exec() {
 
     return 0;
 }
+#endif
 
 }}