build(options): Options propegate much more reliably

Build options such as build-python and build-fortran have much more
predicatable and sensible behavior

src/extern/meson.build

@@ -23,4 +23,7 @@ gridfire_extern_dep = declare_dependency(
 
 install_subdir('include/gridfire', install_dir: get_option('includedir'))
 
-subdir('fortran')
+if get_option('build-fortran')
+    message('Configuring Fortran bindings...')
+    subdir('fortran')
+endif

src/meson.build

@@ -59,6 +59,7 @@ gridfire_dep = declare_dependency(
 
 install_subdir('include/gridfire', install_dir: get_option('includedir'))
 
+message('Configuring C API...')
 subdir('extern')
 
 if get_option('build-python')