Eliminate reliance on /usr/include/aspell.h in aspell existence test

[ryandesign]

The qmake build of TEA only enables aspell support if /usr/include/aspell.h exists:

tea-qt/tea-qmake.pro

Lines 274 to 280 in b4bcaaf

	contains (USE_ASPELL,true) {
	exists ("/usr/include/aspell.h") {
	message ("ASpell enabled")
	LIBS += -laspell
	DEFINES += ASPELL_ENABLE
	}
	}

Ever since aspell support was added to the MacPorts tea port 13 years ago we've been patching that to change /usr to the MacPorts installation location. It would be nicer not to have to patch that. Is there another way that you could detect the existence of aspell that does not rely on its install location? For other feature you use pkg-config and I realize that aspell does not install a pkg-config .pc file, but at least in my installation of aspell 0.60.8 it does have other executables that you might use. For example, it has both aspell and pspell-config executables. Their existence in the current path, or the fact that they execute without error when given some innocuous flag like --version, could be used to infer aspell existence.

[psemiletov]

Consider qmake and aspell support at TEA as legacy. I keep qmake support for old distros with Qt4. Avoid aspell if MacPorts has hunspell. But if you need aspell support without patching, I'll try to fix.