Multi-thread bug of make install

[A1ca7raz]

Hi,
I am packaging this project for Arch Linux riscv64 platform. I found that I failed to make install due to an unexpected error.
It works fine with -j1.

Log:
```
Making install in src
make[1]: Entering directory '/build/libappindicator/src/libappindicator/src'
make install-am
make[2]: Entering directory '/build/libappindicator/src/libappindicator/src'
make[3]: Entering directory '/build/libappindicator/src/libappindicator/src'
 /usr/bin/mkdir -p '/build/libappindicator/pkg/libappindicator-gtk3/usr/include/libappindicator3-0.1/libappindicator'
 /usr/bin/mkdir -p '/build/libappindicator/pkg/libappindicator-gtk3/usr/include/libappindicator3-0.1/libappindicator'
 /usr/bin/mkdir -p '/build/libappindicator/pkg/libappindicator-gtk3/usr/lib/pkgconfig'
 /usr/bin/mkdir -p '/build/libappindicator/pkg/libappindicator-gtk3/usr/share/gir-1.0'
 /usr/bin/mkdir -p '/build/libappindicator/pkg/libappindicator-gtk3/usr/lib'
 /usr/bin/mkdir -p '/build/libappindicator/pkg/libappindicator-gtk3/usr/lib/girepository-1.0'
 /bin/sh ../libtool --mode=install /usr/bin/install -c libappindicator3.la '/build/libappindicator/pkg/libappindicator-gtk3/usr/lib'
 /usr/bin/install -c -m 644 app-indicator.h app-indicator-enum-types.h '/build/libappindicator/pkg/libappindicator-gtk3/usr/include/libappindicator3-0.1/libappindicator'
 /usr/bin/install -c -m 644 app-indicator.h app-indicator-enum-types.h '/build/libappindicator/pkg/libappindicator-gtk3/usr/include/libappindicator3-0.1/libappindicator'
 /usr/bin/install -c -m 644 appindicator3-0.1.pc '/build/libappindicator/pkg/libappindicator-gtk3/usr/lib/pkgconfig'
 /usr/bin/install -c -m 644 AppIndicator3-0.1.gir '/build/libappindicator/pkg/libappindicator-gtk3/usr/share/gir-1.0'
/usr/bin/install: cannot create regular file '/build/libappindicator/pkg/libappindicator-gtk3/usr/include/libappindicator3-0.1/libappindicator/app-indicator.h': File exists
 /usr/bin/install -c -m 644 AppIndicator3-0.1.typelib '/build/libappindicator/pkg/libappindicator-gtk3/usr/lib/girepository-1.0'
make[3]: *** [Makefile:917: install-libappindicatorincludeHEADERS] Error 1
make[3]: *** Waiting for unfinished jobs....
libtool: install: /usr/bin/install -c .libs/libappindicator3.so.1.0.0 /build/libappindicator/pkg/libappindicator-gtk3/usr/lib/libappindicator3.so.1.0.0
libtool: install: (cd /build/libappindicator/pkg/libappindicator-gtk3/usr/lib && { ln -s -f libappindicator3.so.1.0.0 libappindicator3.so.1 || { rm -f libappindicator3.so.1 && ln -s libappindicator3.so.1.0.0 libappindicator3.so.1; }; })
libtool: install: (cd /build/libappindicator/pkg/libappindicator-gtk3/usr/lib && { ln -s -f libappindicator3.so.1.0.0 libappindicator3.so || { rm -f libappindicator3.so && ln -s libappindicator3.so.1.0.0 libappindicator3.so; }; })
libtool: install: /usr/bin/install -c .libs/libappindicator3.lai /build/libappindicator/pkg/libappindicator-gtk3/usr/lib/libappindicator3.la
libtool: install: /usr/bin/install -c .libs/libappindicator3.a /build/libappindicator/pkg/libappindicator-gtk3/usr/lib/libappindicator3.a
libtool: install: chmod 644 /build/libappindicator/pkg/libappindicator-gtk3/usr/lib/libappindicator3.a
libtool: install: ranlib /build/libappindicator/pkg/libappindicator-gtk3/usr/lib/libappindicator3.a
libtool: warning: remember to run 'libtool --finish /usr/lib'
make[3]: Leaving directory '/build/libappindicator/src/libappindicator/src'
make[2]: *** [Makefile:1037: install-am] Error 2
make[2]: Leaving directory '/build/libappindicator/src/libappindicator/src'
make[1]: *** [Makefile:1030: install] Error 2
make[1]: Leaving directory '/build/libappindicator/src/libappindicator/src'
make: *** [Makefile:471: install-recursive] Error 1
==> ERROR: A failure occurred in package_libappindicator-gtk3().
 Aborting...
```