[gparted] Fix make distcheck failure from missing libgtest_main.la (#781978)

[Curtis Gedak <gedakc src gnome org>]

commit dbd5cd2ca80dcc09de9c40878826e8babc4cbc07
Author: Mike Fleetwood <mike fleetwood googlemail com>
Date:   Wed May 10 11:00:59 2017 +0100

    Fix make distcheck failure from missing libgtest_main.la (#781978)
    
    Running 'make distcheck' fails like this:
        [...]
        make[3]: Entering directory `[...]/gparted/gparted-0.28.1-git/_build/tests'
        [...]
        /bin/sh ../libtool --tag=CXX --mode=link g++ [...] ../../lib/gtest/lib/libgtest_main.la 
../../lib/gtest/lib/libgtest.la [...]
        libtool: link: cannot find the library `../../lib/gtest/lib/libgtest_main.la' or unhandled argument 
`../../lib/gtest/lib/libgtest_main.la'
        make[3]: *** [test_dummy] Error 1
        make[3]: Leaving directory `[...]/gparted/gparted-0.28.1-git/_build/tests'
        [...]
        make[1]: Leaving directory `[...]/gparted/gparted-0.28.1-git/_build'
        make: *** [distcheck] Error 1
    
    What is happening is that distcheck [1] performs a VPATH build which
    uses separate read-only source and read-write build trees.  Shipped
    source files, such as .h and .cc, are located in the source tree and the
    built files, such as .o and .la, are compiled into the build tree.  In
    this case the absolute path to libgtest_main.la is specified using
    $(top_srcdir) for the source tree, rather than $(top_builddir) for the
    build tree, hence the cannot find file type error.  This error doesn't
    occur when just performing a 'make check' because that builds in the
    source tree so $(top_builddir) is the same as $(top_srcdir).
    
    Fix by correctly specifying the absolute path to the libgtest*.la files
    using $(top_builddir).
    
    [1] Automake Manual, 14.4 Checking the Distribution
        https://www.gnu.org/software/automake/manual/automake.html#Checking-the-Distribution
    
    Note that for VPATH builds the Automake Manual [2] also recommends using
    a pair of -I options to specify header files are found in both the build
    and source trees, something like:
        AM_CPPFLAGS = -I$(top_builddir)/some/subdir -I$(top_srcdir)/some/subdir
    
    This is unnecessary for GParted as all header files are shipped as part
    of the source archive and no header files are created as part of the
    build process.  Therefore adding -I options specifying $(top_builddir)
    include directories would just be specifying empty or non-existent
    include directories in the build tree.
    
    [2] Automake Manual, 8.7 Variables used when building a program
        https://www.gnu.org/software/automake/manual/automake.html#Program-Variables
    
    Bug 781978 - Add Google Test C++ test framework

 tests/Makefile.am |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)
---
diff --git a/tests/Makefile.am b/tests/Makefile.am
index e347551..dde5c69 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -5,9 +5,9 @@ AM_CPPFLAGS =  \
 AM_CXXFLAGS = -Wall $(GTEST_CXXFLAGS)
 AM_LDFLAGS = $(GTEST_LDFLAGS)
 LDADD =  \
-       $(GTEST_LIBS)                                 \
-       $(top_srcdir)/lib/gtest/lib/libgtest_main.la  \
-       $(top_srcdir)/lib/gtest/lib/libgtest.la
+       $(GTEST_LIBS)                                   \
+       $(top_builddir)/lib/gtest/lib/libgtest_main.la  \
+       $(top_builddir)/lib/gtest/lib/libgtest.la
 
 # Programs to be built by "make check"
 check_PROGRAMS =  \