cppcms: fix build on machines with libgpg-error installed

cppcms: fix build on machines with libgpg-error installed

In configuration where target architecture == host architecture, and
libgpg-error is installed system-wide with development files, the build
of cppcms fails with:

/home/test/buildroot/output/host/usr/bin/x86_64-amd-linux-gnu-g++  --sysroot=/home/test/buildroot/output/host/usr/x86_64-buildroot-linux-gnu/sysroot  -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -Wall -Wextra  -DNDEBUG   CMakeFiles/base64_test.dir/tests/base64_test.cpp.o  -o base64_test  -L/home/test/buildroot/output/host/usr/x86_64-buildroot-linux-gnu/sysroot/usr/lib -Wl,-rpath,/home/test/buildroot/output/build/cppcms-1.0.5:/home/test/buildroot/output/build/cppcms-1.0.5/booster:/usr/lib -rdynamic libcppcms.so.1.0.5 booster/libbooster.so.0.0.3 -lpthread /home/test/buildroot/output/host/usr/x86_64-buildroot-linux-gnu/sysroot/usr/lib/libpcre.so /home/test/buildroot/output/host/usr/x86_64-buildroot-linux-gnu/sysroot/usr/lib/libgcrypt.so /home/test/buildroot/output/host/usr/x86_64-buildroot-linux-gnu/sysroot/usr/lib/libdl.so /home/test/buildroot/output/host/usr/x86_64-buildroot-linux-gnu/sysroot/usr/lib/libz.so
/home/test/buildroot/output/host/usr/x86_64-buildroot-linux-gnu/sysroot/usr/lib/libgcrypt.so: undefined reference to `gpg_err_set_errno@GPG_ERROR_1.0'
/home/test/buildroot/output/host/usr/x86_64-buildroot-linux-gnu/sysroot/usr/lib/libgcrypt.so: undefined reference to `gpgrt_lock_init@GPG_ERROR_1.0'
/home/test/buildroot/output/host/usr/x86_64-buildroot-linux-gnu/sysroot/usr/lib/libgcrypt.so: undefined reference to `gpgrt_lock_destroy@GPG_ERROR_1.0'
/home/test/buildroot/output/host/usr/x86_64-buildroot-linux-gnu/sysroot/usr/lib/libgcrypt.so: undefined reference to `gpg_err_code_from_syserror@GPG_ERROR_1.0'
/home/test/buildroot/output/host/usr/x86_64-buildroot-linux-gnu/sysroot/usr/lib/libgcrypt.so: undefined reference to `gpg_err_code_from_errno@GPG_ERROR_1.0'
/home/test/buildroot/output/host/usr/x86_64-buildroot-linux-gnu/sysroot/usr/lib/libgcrypt.so: undefined reference to `gpgrt_lock_unlock@GPG_ERROR_1.0'
/home/test/buildroot/output/host/usr/x86_64-buildroot-linux-gnu/sysroot/usr/lib/libgcrypt.so: undefined reference to `gpg_strerror@GPG_ERROR_1.0'
/home/test/buildroot/output/host/usr/x86_64-buildroot-linux-gnu/sysroot/usr/lib/libgcrypt.so: undefined reference to `gpg_strsource@GPG_ERROR_1.0'
/home/test/buildroot/output/host/usr/x86_64-buildroot-linux-gnu/sysroot/usr/lib/libgcrypt.so: undefined reference to `gpgrt_lock_lock@GPG_ERROR_1.0'

The problem comes from the
"-Wl,-rpath,/home/test/buildroot/output/build/cppcms-1.0.5:/home/test/buildroot/output/build/cppcms-1.0.5/booster:/usr/lib"
option, which tells the linker to search for libraries in /usr/lib.

This commit fixes that by asking CMake to not add any rpath when
building cppcms.

Fixes:

  http://autobuild.buildroot.net/results/a7eb1ede552ae14f409cfd7bd877bcf25ca69a74/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>