Disable position independent code for the executable target.

Upstream enables CMAKE_POSITION_INDEPENDENT_CODE globally in
cmake/macros.cmake (for the benefit of static libraries that get linked
into shared libraries), which propagates -fPIE to the wxFormBuilder_app
executable. Gentoo's tinderbox QA flags this as an "uncommon" flag
(QA0069), since PIE handling belongs to the toolchain. Set the property
to OFF on the executable target so libs still get -fPIC.

https://bugs.gentoo.org/948602
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -16,6 +16,7 @@
 add_executable(wxFormBuilder_app WIN32 MACOSX_BUNDLE)
 add_executable(wxFormBuilder::app ALIAS wxFormBuilder_app)
 set_target_properties(wxFormBuilder_app PROPERTIES
+  POSITION_INDEPENDENT_CODE OFF
   OUTPUT_NAME "$<IF:$<OR:$<BOOL:${WIN32}>,$<BOOL:${APPLE}>>,wxFormBuilder,wxformbuilder>"
 )