Compile libxcb on Linux

[radarhere]

Resolves python-pillow/Pillow#4558

Trying to compile libxcb on Linux, I encountered

configure: error: Package requirements (xcb-proto >= 1.14) were not met:

Variable 'pc_sysrootdir' not defined in '/usr/local/lib/pkgconfig/xcb-proto.pc'

Consider adjusting the PKG_CONFIG_PATH environment variable if you
installed software in a non-standard prefix.

Alternatively, you may set the environment variables XCBPROTO_CFLAGS
and XCBPROTO_LIBS to avoid the need to call pkg-config.
See the pkg-config man page for more details.

Printing /usr/local/lib/pkgconfig/xcb-proto.pc, I found

prefix=/usr/local
exec_prefix=${prefix}
datarootdir=${prefix}/share
datadir=${datarootdir}
libdir=${exec_prefix}/lib
xcbincludedir=${pc_sysrootdir}${datadir}/xcb
pythondir=${pc_sysrootdir}${prefix}/lib/python3.5/site-packages

Name: XCB Proto
Description: X protocol descriptions for XCB
Version: 1.14

Looking elsewhere in the Travis log, it can be seen that the full path to site-packages is /usr/local/lib/python3.5/site-packages. So ${pc_sysrootdir} is empty for our setup, and I've just added a line to remove it with sed.

Why did this error occur on Linux and not on macOS? I would guess it is because they are running different versions of pkg-config. When I run pkg-config --version on TravisCI macOS, I get 0.29.2. On TravisCI Linux, I get 0.21.

[hugovk]

Thanks!