fix splash loading with wx3.0

Previously only gray square was shown. Apparently to show splash screen several calls of wx.Yield() are necessary.
Unfortunately splash.IsShowOnScreen() returns always 1 regardless of whether splash is on the screen or not.
this fix is a dirty hack. It just process all events after splash is created during 0.3 seconds.

from ..toolchain_gcc import toolchain_gcc

class Linux_target(toolchain_gcc):
    dlopen_prefix = "./"
    extension = ".so"
    def getBuilderCFLAGS(self):
        return toolchain_gcc.getBuilderCFLAGS(self) + ["-fPIC"]
    def getBuilderLDFLAGS(self):
        return toolchain_gcc.getBuilderLDFLAGS(self) + ["-shared", "-lrt"]