py-meson: updated to 0.51.0

0.51.0:
(C) Preprocessor flag handling
Sanity checking compilers with user flags
New sourceset module
n_debug=if-release and buildtype=plain means no asserts
target_type in build_targets accepts the value 'shared_module'
New modules kwarg for python.find_installation
Support for the Intel Compiler on Windows (ICL)
Added basic support for the Xtensa CPU toolchain
Dependency objects now have a get_variable method
CMake prefix path overrides
Tests that should fail but did not are now errors
New target keyword argument: link_language
New module to parse kconfig output files
Add new meson subprojects foreach command
Added c17 and c18 as c_std values for recent GCC and Clang Versions
gpgme dependency now supports gpgme-config
Can link against custom targets
Removed the deprecated --target-files API
Generators have a new depends keyword argument
Specifying options per mer machine
subproject.get_variable() now accepts a fallback argument
Add keyword static to find_library
Fortran include statements recursively parsed
CMake subprojects
Multipe cross files can be specified

(adam)