Cross compilation of GStreamer in Ubuntu docker container resulting in not all subprojects being built

[Ben Rush]

In particular, I'm having trouble getting the webrtc library to build, and
it appears as though -- if I track it back far enough -- the issue is with
libnice. The meson build output results in the output at the end of this
email (I'll defer posting it now to make the email reading easier).

According to the error, the problem is that pkg-config is unable to find
openssl. But running this at the command prompt (after attaching to the
container), results in what appears to be a positive for finding openssl:

root at b5f46e708137:/gst-build# pkg-config --version openssl
0.29.1

Ultimately I'm curious if someone has been able to build webrtc and the
like cross compiled for Windows from Ubuntu, etc. I'd love to have a nice,
dockerized system for building for Windows but after about day of fighting,
I've yet to get it to work completely.

Here is the output:

|Executing subproject libnice method meson
|
|Project name: libnice
|Project version: 0.1.17.1
|C compiler for the build machine: cc (gcc 7.5.0 "cc (Ubuntu
7.5.0-3ubuntu1~18.04) 7.5.0")
|C linker for the build machine: cc ld.bfd 2.30
|C compiler for the host machine: x86_64-w64-mingw32-gcc (gcc 7.3.0
"x86_64-w64-mingw32-gcc (GCC) 7.3-win32 20180312")
|C linker for the host machine: x86_64-w64-mingw32-gcc ld.bfd 2.30
|Library iphlpapi found: YES
|Library ws2_32 found: YES
|Checking for function "clock_gettime" : NO (cached)
|Library rt found: NO
|Checking for function "clock_gettime" with dependency -lrt: NO (cached)
|Has header "arpa/inet.h" : NO (cached)
|Has header "net/in.h" : NO
|Has header "netdb.h" : NO (cached)
|Has header "ifaddrs.h" : NO
|Has header "unistd.h" : YES (cached)
|Checking for function "poll" : NO (cached)
|Checking for function "getifaddrs" : NO
|Compiler for C supports arguments -fno-strict-aliasing: YES
|Message: warning level: 1
|Message: werror enabled: false
|Dependency gio-2.0 found: YES 2.62.6 (cached)
|Dependency (anonymous) from subproject subprojects/glib found: YES 2.62.6
|Dependency (anonymous) from subproject subprojects/glib found: YES 2.62.6
|Dependency (anonymous) from subproject subprojects/glib found: YES 2.62.6
|Dependency gthread-2.0 from subproject subprojects/glib found: YES 2.62.6
|Message: Crypto library requested: auto
|Run-time dependency gnutls found: NO (tried pkgconfig and cmake)
|Run-time dependency openssl found: NO (tried pkgconfig and cmake)
|Looking for a fallback subproject for the dependency openssl
|Subproject directory not found and openssl.wrap file not found
|Subproject  subprojects/openssl is buildable: NO (disabling)
|Dependency openssl from subproject subprojects/openssl found: NO
|Library crypto found: NO
|Library eay32 found: NO
|
|subprojects/libnice/meson.build:249:4: Exception: Problem encountered:
Either GnuTLS or OpenSSL is required as crypto library, but neither was
found
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