Fix bug #7954 (now really)

Thanks to maciejr we know now what the remaining problem was with bug 7954:
My unicode symbol fallback works fine, the problem was that a font named
"Symbol" is available on OS X, but it does not use the font-specific encoding
we expect: Almost all glyphs are at their unicode code point.
Therefore the bug is fixed by re-enabling the unicode workaround and blocking
the Symbol font on OS X.

lib/symbols

@@ -1173,6 +1173,8 @@ lyxsurd               cmsy        112 0 mathord  √
 \def\surd{^\lyxsurd}
 \def\textdegree{\kern-1mu^{\circ}\kern-4mu}
 else
+# FIXME: These don't work on OS X, since the Symbol font uses a different
+#        encoding and is therefore disabled in FontLoader::available().
 cong               lyxsymbol             64  0 mathrel  ≅
 surd               lyxsymbol             214 0 mathord  √
 textdegree         lyxsymbol             176 0 mathord  °

src/frontends/qt4/GuiFontLoader.cpp

@@ -343,6 +343,16 @@ bool FontLoader::available(FontInfo const & f)
 	static vector<int> cache(NUM_FAMILIES, false);
 
 	FontFamily family = f.family();
+#ifdef Q_WS_MACX
+	// Apple ships a font name "Symbol", which has more or less the same
+	// glyphs as the original PostScript Symbol font, but it uses a different
+	// encoding (see https://en.wikipedia.org/wiki/Symbol_(typeface)#cite_note-2).
+	// Since we expect the font specific encoding of the original
+	// PostScript Symbol font, we can't use the one provided on OS X.
+	// See also the discussion in bug 7954.
+	if (f.family() == SYMBOL_FAMILY)
+		return false;
+#endif
 	if (cache_set[family])
 		return cache[family];
 	cache_set[family] = true;

src/mathed/MathFactory.cpp

@@ -118,10 +118,8 @@ bool canBeDisplayed(char_type c)
 }
 
 
-bool isUnicodeSymbolAvailable(docstring const & /*name*/, char_type & /*c*/)
+bool isUnicodeSymbolAvailable(docstring const & name, char_type & c)
 {
-	// this is too fragile, e.g. prodes W instead of capital omega on OS X
-#if 0
 	docstring cmd(from_ascii("\\") + name);
 	bool is_combining;
 	bool termination;
@@ -135,9 +133,6 @@ bool isUnicodeSymbolAvailable(docstring const & /*name*/, char_type & /*c*/)
 		                                is_combining, termination);
 	}
 	return c != 0 && !is_combining;
-#else
-	return false;
-#endif
 }
 
 

src/mathed/MathSupport.cpp

@@ -635,6 +635,9 @@ fontinfo fontinfos[] = {
 	// LyX internal usage
 	{"lyxtex",        inh_family, inh_series,
 			  UP_SHAPE, Color_latex},
+	// FIXME: The following two don't work on OS X, since the Symbol font
+	//        uses a different encoding, and is therefore disabled in
+	//        FontLoader::available().
 	{"lyxsymbol",     SYMBOL_FAMILY, inh_series,
 			  inh_shape, Color_math},
 	{"lyxboldsymbol", SYMBOL_FAMILY, BOLD_SERIES,