Musl-libc (https://www.musl-libc.org/) is a glibc alternative that developed in sense of to be correct, lightweight and fast. Some Linux Distributions use it as default instead of glibc (like Alpine Linux) and some others offer it as a flavor. Open source applications can be compiled and run on musl without hassle but this situation is different with some binary-only applications like Urban Terror.
Since UrbanTerror releases build on a glibc host, it contains some (actually only one) glibc specific symbol that prevents it to be launched on musl based distributions successfully.
Here is ldd output of 4.3 binary on Alpine Linux 3.5.2:
$ ldd Quake3-UrT.x86_64 /lib64/ld-linux-x86-64.so.2 (0x5630b4353000) libSDL-1.2.so.0 => /usr/lib/libSDL-1.2.so.0 (0x7fed4ba68000) libpthread.so.0 => /lib64/ld-linux-x86-64.so.2 (0x5630b4353000) libX11.so.6 => /usr/lib/libX11.so.6 (0x7fed4b745000) libXxf86vm.so.1 => /usr/lib/libXxf86vm.so.1 (0x7fed4b540000) libdl.so.2 => /lib64/ld-linux-x86-64.so.2 (0x5630b4353000) libm.so.6 => /lib64/ld-linux-x86-64.so.2 (0x5630b4353000) libc.so.6 => /lib64/ld-linux-x86-64.so.2 (0x5630b4353000) libsoxr.so.0 => /usr/lib/libsoxr.so.0 (0x7fed4b2d3000) libxcb.so.1 => /usr/lib/libxcb.so.1 (0x7fed4b0ad000) libXext.so.6 => /usr/lib/libXext.so.6 (0x7fed4ae9c000) libgomp.so.1 => /usr/lib/libgomp.so.1 (0x7fed4ac7a000) libXau.so.6 => /usr/lib/libXau.so.6 (0x7fed4aa77000) libXdmcp.so.6 => /usr/lib/libXdmcp.so.6 (0x7fed4a871000) Error relocating Quake3-UrT.x86_64: __strtod_internal: symbol not found
It seems that if Urban Terror developers can find another way to accommodate "__strtod_internal" macro, we musl users will be able play Urban Terror on our favorite distributions. Hope this problem can be fixed without too much hassle with both glibc+musl compatible releases.