pycairo fails to build because prerequisite Python.h is not available
gst-build shouldn't bail out if non-essential stuff like Python.h is not available.
It should skip non-core subprojects if stuff isn't available:
[193/3563] Compiling C object 'subprojects/pycairo/cairo/3e19734@@_cairo.cpython-37m-x86_64-linux-gnu@sha/bufferproxy.c.o'.
FAILED: subprojects/pycairo/cairo/3e19734@@_cairo.cpython-37m-x86_64-linux-gnu@sha/bufferproxy.c.o
cc -Isubprojects/pycairo/cairo/3e19734@@_cairo.cpython-37m-x86_64-linux-gnu@sha -Isubprojects/pycairo/cairo -I../../home/admin/gst-build/subprojects/pycairo/cairo -I/usr/include/cairo -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I/usr/include/pixman-1 -I/usr/include/uuid -I/usr/include/freetype2 -I/usr/include/libpng16 -fdiagnostics-color=always -pipe -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -O2 -g -fPIC -DPYCAIRO_VERSION_MAJOR=1 -DPYCAIRO_VERSION_MINOR=18 -DPYCAIRO_VERSION_MICRO=1 -Wall -Warray-bounds -Wcast-align -Wconversion -Wdeclaration-after-statement -Wextra -Wformat=2 -Wformat-nonliteral -Wformat-security -Wimplicit-function-declaration -Winit-self -Winline -Wmissing-format-attribute -Wmissing-noreturn -Wnested-externs -Wold-style-definition -Wpacked -Wpointer-arith -Wreturn-type -Wshadow -Wsign-compare -Wstrict-aliasing -Wundef -Wunused-but-set-variable -Wno-missing-field-initializers -Wno-unused-parameter -fno-strict-aliasing -fvisibility=hidden -Wswitch-default -MD -MQ 'subprojects/pycairo/cairo/3e19734@@_cairo.cpython-37m-x86_64-linux-gnu@sha/bufferproxy.c.o' -MF 'subprojects/pycairo/cairo/3e19734@@_cairo.cpython-37m-x86_64-linux-gnu@sha/bufferproxy.c.o.d' -o 'subprojects/pycairo/cairo/3e19734@@_cairo.cpython-37m-x86_64-linux-gnu@sha/bufferproxy.c.o' -c ../../home/admin/gst-build/subprojects/pycairo/cairo/bufferproxy.c
../../home/admin/gst-build/subprojects/pycairo/cairo/bufferproxy.c:32:10: fatal error: Python.h: No such file or directory
#include <Python.h>