google-benchmark: build as a static library

This partially fixes the Windows build, which was broken
because the upstream package does not use Visual Studio
specific code annotations like __dllspec(dllexport) or any
other mechanism for exporting symbols.

There are two other reasons to fix it this way:

Upstream uses CMake's WINDOWS_EXPORT_ALL_SYMBOLS feature to
deal with the issue for DLL builds. Meson does not have a
similar feature (see
mesonbuild/meson#2132).

Google typically links benchmarks statically, so linking
statically even on non-Windows builds is closer to the way
upstream typically uses the library.

subprojects/packagefiles/google-benchmark/meson.build

@@ -30,7 +30,7 @@ inc = include_directories('include')
 
 thread_dep = dependency('threads')
 
-lib_benchmark = library('gbenchmark',
+lib_benchmark = static_library('gbenchmark',
                         benchmark_sources,
                         include_directories:inc,
                         cpp_args : '-DNDEBUG',
@@ -40,7 +40,7 @@ google_benchmark_dep = declare_dependency(
                         link_with:lib_benchmark,
                         include_directories:inc)
 
-lib_benchmark_main = library('benchmark-main',
+lib_benchmark_main = static_library('benchmark-main',
                         benchmark_main_sources,
                         include_directories:inc,
                         cpp_args : '-DNDEBUG',