cmake: Use CMAKE_BUILD_TYPE flags in musllib

These flags include debug and optimization defaults for different build
types Release, Debug, RelWithDebInfo, and MinSizeRel. These already get
added to targets built within CMake but aren't automatically inherited
by external projects.

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index f685da2..01fc6de 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -76,7 +76,14 @@
 
 # We make an attempt to extract compiler options from CMake
 get_property(compile_options DIRECTORY PROPERTY COMPILE_OPTIONS)
-separate_arguments(cmake_c_flags_sep NATIVE_COMMAND "${CMAKE_C_FLAGS}")
+
+# Also inherit the CMAKE build type flags from one of the following variables depending on
+# the configured build type:
+# CMAKE_C_FLAGS_DEBUG, CMAKE_C_FLAGS_RELEASE, CMAKE_C_FLAGS_RELWITHDEBINFO, CMAKE_C_FLAGS_MINSIZEREL
+string(TOUPPER CMAKE_BUILD_TYPE build_type)
+set(cmake_built_type_config_name CMAKE_C_FLAGS_${build_type})
+separate_arguments(cmake_c_flags_sep NATIVE_COMMAND "${CMAKE_C_FLAGS};${${cmake_built_type_config_name}}")
+
 list(APPEND compile_options "${cmake_c_flags_sep}")
 
 # Add the target triple to the compile flags if we are using clang