Bug#788776: FTBFS: DSO missing from command line: XInitThreads

[Chris West (Faux)]

Source: mathgl
Version: 2.2.2.1-3
Severity: serious
Justification: fails to build from source (but built successfully in the past)

Dear Maintainer,

The package fails to build from source due to a missing linker argument (-lx11):

/usr/bin/ld: CMakeFiles/udav.dir/udav_wnd.cpp.o: undefined reference to symbol 'XInitThreads'
//usr/lib/x86_64-linux-gnu/libX11.so.6: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
udav/CMakeFiles/udav.dir/build.make:1395: recipe for target 'udav/udav' failed
make[3]: *** [udav/udav] Error 1

The full build logs can be seen on the Reproducible Builds builder:
https://reproducible.debian.net/rb-pkg/unstable/amd64/mathgl.html

-- System Information:
Debian Release: stretch/sid
  APT prefers unstable
  APT policy: (500, 'unstable'), (1, 'experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 3.19.0-20-generic (SMP w/8 CPU cores)
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)