docs: remove specific clang version from parameters

docs/building.md

 To use a different compiler, or compiler flags, you can specify these before
 the `configure` command. For example:
 
-	CC=clang-3.5 CFLAGS=-Wextra ./configure --prefix=/usr
+	CC=clang CFLAGS=-Wextra ./configure --prefix=/usr
 
 The `espeak-ng` and `speak-ng` programs, along with the espeak-ng voices, can
 then be built with:
 
 	CFLAGS="-fsanitize=address,undefined -g" \
 		LDFLAGS="-fsanitize=address,undefined" \
-		CC=clang-6.0 ./configure
+		CC=clang ./configure
 	make
 	make check
 
 To enable libFuzzer support you need clang 6.0 or later. It is enabled with
 the following:
 
-	CC=clang-6.0 ./configure --with-libfuzzer=yes
+	CC=clang ./configure --with-libfuzzer=yes
 	make
 	make check