Update build requirements in el6 spec file

Make the dependency on g++ explicit.

Now that the native version of cmake in el6 has been bumped to
v2.8 we can remove the dependency on EPEL's cmake28 package.
diff --git a/contrib/packages/rpm/el6/SPECS/tigervnc.spec b/contrib/packages/rpm/el6/SPECS/tigervnc.spec
index 1e846b5..9754c08 100644
--- a/contrib/packages/rpm/el6/SPECS/tigervnc.spec
+++ b/contrib/packages/rpm/el6/SPECS/tigervnc.spec
@@ -22,6 +22,7 @@
 Source17: ftp://ftp.gnutls.org/gcrypt/gnutls/v3.3/gnutls-3.3.13.tar.xz
 BuildRoot: %{_tmppath}/%{name}-%{version}-%{release}-root-%(%{__id_u} -n)
 
+BuildRequires: gcc, gcc-c++
 BuildRequires: libX11-devel, automake, autoconf, libtool, gettext, gettext-devel
 BuildRequires: libXext-devel, xorg-x11-server-source, libXi-devel
 BuildRequires: xorg-x11-xtrans-devel, xorg-x11-util-macros, libXtst-devel
@@ -31,7 +32,7 @@
 BuildRequires: freetype-devel, libXdmcp-devel
 BuildRequires: desktop-file-utils, java-devel, jpackage-utils
 BuildRequires: libjpeg-turbo-devel, pam-devel
-BuildRequires: cmake28
+BuildRequires: cmake >= 2.8
 %ifnarch s390 s390x
 BuildRequires: xorg-x11-server-devel
 %endif
@@ -261,7 +262,7 @@
 make DESTDIR=%{static_lib_buildroot} install
 popd
 
-%{cmake28} -G"Unix Makefiles" \
+%{cmake} -G"Unix Makefiles" \
   -DBUILD_STATIC=off \
   -DCMAKE_INSTALL_PREFIX=%{_prefix} \
   -DFLTK_LIBRARIES="%{static_lib_buildroot}%{_libdir}/libfltk.a;%{static_lib_buildroot}%{_libdir}/libfltk_images.a;%{static_lib_buildroot}%{_libdir}/libpng.a" \
@@ -301,7 +302,7 @@
 
 # Build Java applet
 pushd java
-%{cmake28} \
+%{cmake} \
 %if !%{_self_signed}
 	-DJAVA_KEYSTORE=%{_keystore} \
 	-DJAVA_KEYSTORE_TYPE=%{_keystore_type} \