src/CMakeLists.txt: add an option to disable plugins build
Plugins build need the "lv2-ttl-generator" tool. When cross compiling,
the tool binary execution will fail, stopping the build. This patch
add an option to disable plugins to avoid the issue. Cross compilation
is mostly needed for embedded systems. In this case, only the core
binary without GUI is generally needed, without plugins.
To preserve the original behavior, plugins are enabled by default.
Signed-off-by: Julien Olivain <ju.o@free.fr>