Update meson.build to respect default_library for libb64

Allows building a fully static libcper when default_library=static

Change-Id: I5790a53c38609fe2b92614775f0da100e140dd77
Signed-off-by: Andrew Adriance <aadriance@nvidia.com>
diff --git a/meson.build b/meson.build
index b34dab1..80d46b2 100644
--- a/meson.build
+++ b/meson.build
@@ -12,6 +12,8 @@
     ],
 )
 
+library_is_share = get_option('default_library') == 'shared'
+
 project_description = 'libcper library'
 
 SectionSources = files(
@@ -61,7 +63,7 @@
     opt_var = cmake.subproject_options()
     opt_var.add_cmake_defines(
         {
-            'BUILD_SHARED_LIBS': true,
+            'BUILD_SHARED_LIBS': library_is_share,
             'BASE64_BUILD_CLI': false,
             'BASE64_WITH_AVX': false,
             'BASE64_WITH_AVX2': false,
@@ -72,7 +74,6 @@
         },
     )
 
-
     libb64_ex = cmake.subproject('libb64', options: opt_var)
     libb64 = libb64_ex.dependency('base64')
 endif