Add iOS CMake build to Travis CI

Use the following CMake flags:
-DCMAKE_SYSTEM_NAME=iOS - for specifying it is an iOS build
-DCMAKE_FIND_ROOT_PATH=/usr/local - dir to search for the built wxWidgets libraries
-DCMAKE_XCODE_ATTRIBUTE_CODE_SIGNING_ALLOWED=NO - don't require signing app packages
-DCMAKE_OSX_DEPLOYMENT_TARGET=10.12 - specify a target so both the libraries and sample will use the same architecture

Also use the code signing flag when testing precompiled header support.
Don't build the default samples, widgets sample has features that are not supported by the iOS build.

Xcode seems to rebuild the libraries when installing, so call CMake only once.

build/cmake/init.cmake

@@ -543,17 +543,23 @@ endif()
 set(wxBUILD_PRECOMP_PREV ${wxBUILD_PRECOMP} CACHE INTERNAL "")
 
 if(wxBUILD_PRECOMP)
+    if(DEFINED CMAKE_XCODE_ATTRIBUTE_CODE_SIGNING_ALLOWED)
+        set(try_flags "-DCMAKE_XCODE_ATTRIBUTE_CODE_SIGNING_ALLOWED=${CMAKE_XCODE_ATTRIBUTE_CODE_SIGNING_ALLOWED}")
+    endif()
     if (CLEAN_PRECOMP_TEST)
         try_compile(RESULT_VAR_CLEAN
                     "${wxBINARY_DIR}/CMakeFiles/cotire_test"
                     "${wxSOURCE_DIR}/build/cmake/modules/cotire_test"
                     CotireExample clean_cotire
+                    CMAKE_FLAGS ${try_flags}
         )
     endif()
     try_compile(RESULT_VAR
                 "${wxBINARY_DIR}/CMakeFiles/cotire_test"
                 "${wxSOURCE_DIR}/build/cmake/modules/cotire_test"
-                CotireExample OUTPUT_VARIABLE OUTPUT_VAR
+                CotireExample
+                CMAKE_FLAGS ${try_flags}
+                OUTPUT_VARIABLE OUTPUT_VAR
     )
 
     # check if output has precompiled header warnings. The build can still succeed, so check the output