build: Remove unsupported install directives

We're using the `install` argument for configure_file() all over the
place.

The support for an `install` argument for configure_file() was added in
Meson 0.50, but we haven't bumped the minimum version of Meson we
require, yet; which means we're getting compatibility warnings when
using recent versions of Meson, and undefined behaviour when using older
versions.

The configure_file() object defaults to `install: false`, unless an
install directory is used. This means that all instances of an `install`
argument with an explicit `true` or `false` value can be removed,
whereas all instances of `install` with a value determined from a
configuration option must be turned into an explicit conditional.

gobject/meson.build

@@ -75,7 +75,6 @@ foreach tool: python_tools
     input : tool + '.in',
     output : tool,
     configuration : python_tools_conf,
-    install : true,
     install_dir : glib_bindir,
   )
 
@@ -163,11 +162,11 @@ if enable_systemtap
     output : '@0@.stp'.format(libgobject.full_path().split('/').get(-1)),
     configuration : stp_cdata,
     install_dir : tapset_install_dir,
-    install : true)
+  )
 endif
 
 # Don’t build the tests unless we can run them (either natively or in an exe wrapper)
 build_tests = not meson.is_cross_build() or (meson.is_cross_build() and meson.has_exe_wrapper())
 if build_tests
   subdir('tests')
-endif
+endif