port-to-gtk3: Switch to Gtkmm3 (#7)

Switch to Gtkmm3 in configure.ac.  Require version 3.0.0.

Also update the build instructions in README and the package list in
.gitlab-ci.yml.

Starting from version 3.18.0 Gtkmm requires C++11 compilation [1][2][3].
Add a check for Gtkmm >= 3.18.0 in configure.ac and enable C++11
compilation accordingly.

References:

[1] Gtkmm 3.18.1 NEWS
    "Changes in 3.18 ... Use, and require C++11, ..."
    https://gitlab.gnome.org/GNOME/gtkmm/blob/3.18.1/NEWS#L35

[2] Murray's Blog - "gtkmm now uses C++11"
    https://www.murrayc.com/permalink/2015/07/31/gtkmm-now-uses-c11

[3] Murray's Blog - "gtkmm 3.18 and glibmm 2.46"
    https://www.murrayc.com/permalink/2015/09/25/gtkmm-3-18-and-glibmm-2-46

Closes #7 - Port to Gtk3

.gitlab-ci.yml

@@ -8,7 +8,7 @@ stages:
   before_script:
     - yum update -y
     - yum install -y which gnome-common gnome-doc-utils glib2-devel intltool
-                     gcc-c++ libuuid-devel parted-devel gtkmm24-devel make
+                     gcc-c++ libuuid-devel parted-devel gtkmm30-devel make
                      polkit file rarian-compat
 
 .ubuntu_image_template: &ubuntu_image_definition
@@ -17,7 +17,7 @@ stages:
   before_script:
     - apt-get update
     - apt-get install -y gnome-common gnome-doc-utils libglib2.0-dev-bin
-                         uuid-dev libparted-dev libgtkmm-2.4-dev make
+                         uuid-dev libparted-dev libgtkmm-3.0-dev make
                          policykit-1 rarian-compat
 
 .build_stage_template: &build_stage_definition

README

@@ -86,7 +86,7 @@ b. Building from Source
       e2fsprogs
       parted
       gnome-common
-      gtkmm24
+      gtkmm3
       gettext
       gnome-doc-utils     - required if help documentation is to be built
 
@@ -96,11 +96,11 @@ b. Building from Source
       sudo apt-get build-dep gparted
      Or;
       sudo apt-get install build-essential e2fsprogs uuid uuid-dev \
-                           gnome-common libparted-dev libgtkmm-2.4-dev \
+                           gnome-common libparted-dev libgtkmm-3.0-dev \
                            libdevmapper-dev gnome-doc-utils docbook-xml
 
    On Fedora, you will need to run (as root);
-      yum install gtkmm24-devel parted-devel e2fsprogs-devel gettext \
+      yum install gtkmm30-devel parted-devel e2fsprogs-devel gettext \
                   'perl(XML::Parser)' desktop-file-utils libuuid-devel \
                   gnome-doc-utils docbook-dtds intltool gnome-common \
                   gcc-c++
@@ -109,7 +109,7 @@ b. Building from Source
    On openSUSE, these dependencies may be obtained by running the
    following commands;
       sudo zypper install automake autoconf make gnome-common \
-                          libuuid-devel parted-devel gtkmm2-devel \
+                          libuuid-devel parted-devel gtkmm3-devel \
                           gnome-doc-utils-devel docbook-xsl-stylesheets
       sudo zypper install -t pattern devel_c_c++
 

configure.ac

@@ -199,7 +199,7 @@ AC_SUBST([GTHREAD_LIBS])
 AC_SUBST([GTHREAD_CFLAGS])
 
 dnl GTKMM
-PKG_CHECK_MODULES([GTKMM], [gtkmm-2.4 >= 2.24.0])
+PKG_CHECK_MODULES([GTKMM], [gtkmm-3.0 >= 3.0.0])
 AC_SUBST([GTKMM_LIBS])
 AC_SUBST([GTKMM_CFLAGS])
 
@@ -227,6 +227,15 @@ PKG_CHECK_EXISTS(
 	],
 	[AC_MSG_RESULT([no])]
 )
+dnl Check for gtkmm >= 3.18.0 which requires C++11 compilation.
+AC_MSG_CHECKING([for gtkmm >= 3.18.0 which requires C++11 compilation])
+PKG_CHECK_EXISTS(
+	[gtkmm-3.0 >= 3.18.0],
+	[need_cxx_compile_stdcxx_11=yes
+	 AC_MSG_RESULT([yes])
+	],
+	[AC_MSG_RESULT([no])]
+)
 dnl Enable C++11 compilation only if required.
 if test "x$need_cxx_compile_stdcxx_11" = xyes; then
 	AX_CXX_COMPILE_STDCXX_11()