Install gpartedbin into @libexecdir@ (#85)

Executables which are not intended for execution by users, but by other
programs, should be installed into /usr/libexec [1][2].  gpartedbin
falls into this category.  Update it's installation accordingly.

Standard Autotools details: gpartedbin will be installed into
EPREFIX/libexec by default.  To install gpartedbin into a different
directory set libexecdir when configuring the build system.  Like this
from git:
    ./autogen.sh --libexecdir=DIR
or like this from tar release:
    ./configure --libexecdir=DIR

[1] Filesystem Hierarchy Standard, version 3.0,
    4.7. /usr/libexec : Binaries run by other programs (optional)
    https://refspecs.linuxfoundation.org/FHS_3.0/fhs/ch04s07.html
    "/usr/libexec includes internal binaries that are not intended to be
    executed directly by users or shell scripts.
    "

[2] GNU Coding Standards, June 12, 2020,
    7.2.5 Variables for Installation Directories
    https://www.gnu.org/prep/standards/html_node/Directory-Variables.html
    "libexecdir
    The directory for installing executable programs to be run by other
    programs rather than by users.  This directory should normally be
    /usr/local/libexec, but write it as $(exec_prefix)/libexec.  (If you
    are using Autoconf, write it as '@libexecdir@'.)
    "

Closes #85 - Please install gpartedbin under /usr/libexec instead of
             /usr/sbin

Makefile.am

@@ -32,7 +32,7 @@ endif
 bin_SCRIPTS = gparted
 CLEANFILES = $(bin_SCRIPTS) $(DESKTOP_IN_FILES) $(polkit_action_in_FILES)
 
-do_subst = sed -e 's,[@]sbindir[@],$(sbindir),g' \
+do_subst = sed -e 's,[@]libexecdir[@],$(libexecdir),g' \
 	-e 's,[@]bindir[@],$(bindir),g' \
 	-e 's,[@]gksuprog[@],$(GKSUPROG),g' \
 	-e 's,[@]enable_xhost_root[@],$(ENABLE_XHOST_ROOT),g'

gparted.in

@@ -39,7 +39,7 @@ fi
 #
 #  Define base command for executing GParted
 #
-BASE_CMD="@sbindir@/gpartedbin $*"
+BASE_CMD="@libexecdir@/gpartedbin $*"
 
 #
 #  For non-root users try to get authorisation to run GParted as root.

src/Makefile.am

@@ -8,7 +8,7 @@ AM_CFLAGS = -Wall
 
 AM_CXXFLAGS = -Wall	
 
-sbin_PROGRAMS = gpartedbin
+libexec_PROGRAMS = gpartedbin
 
 gpartedbin_SOURCES = \
 	BlockSpecial.cc			\