README.freebsd: remove no longer applicable GCC comment

FreeBSD used to ship an old version of GCC in the base system but this
is not the case for any supported version (and has not been the case for
many years).  Update the note in README.freebsd to reflect this.

README.freebsd

@@ -21,9 +21,8 @@ fswatch.
 
 fswatch is a C++ program and a C++ compiler compliant with the C++11 standard is
 required to compile it.  FreeBSD v. >= 10.0 ships by default with the CLang
-compiler suite which builds this package correctly.  The GCC compiler collection
-shipped with FreeBSD, however, is not C++11 compliant and cannot build this
-package.
+compiler suite which builds this package correctly.  It can also be built with
+GCC installed from the package collection or ports tree.
 
 The configure script enforces an ordered compiler search list and clang++ will
 be used first if available.  If you do not like this choice and wish to use
@@ -46,4 +45,4 @@ WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
 PARTICULAR PURPOSE.  See the GNU General Public License for more details.
 
 You should have received a copy of the GNU General Public License along with
-this program.  If not, see <http://www.gnu.org/licenses/>.
+this program.  If not, see <http://www.gnu.org/licenses/>.