rtloader: Use execinfo only on glibc (#15256)

Use execinfo only on glibc.
Functions in execinfo.h are GNU extensions and not available on other C libraries like musl.

We used to use libexecinfo package (A quick-n-dirty BSD licensed clone of the GNU libc backtrace facility.) of Alpine Linux to build datadog-agent on Alpine, but it has been removed since Alpine 3.17.
This PR allow to build datadog-agent on Alpine Linux and other non-glibc environments.

releasenotes/notes/rtloader-use-execinfo-only-on-glibc-5b734df32fc1555a.yaml

@@ -0,0 +1,13 @@
+# Each section from every release note are combined when the
+# CHANGELOG.rst is rendered. So the text needs to be worded so that
+# it does not depend on any information only available in another
+# section. This may mean repeating some details, but each section
+# must be readable independently of the other.
+#
+# Each section note must be formatted as reStructuredText.
+---
+fixes:
+  - |
+    rtloader: Use `execinfo` only if provided to fix builds on
+    C libraries like `musl`.
+

rtloader/rtloader/CMakeLists.txt

@@ -32,13 +32,15 @@ add_library(datadog-agent-rtloader SHARED
 find_library(EXECINFO execinfo)
 if(EXECINFO)
     target_link_libraries(datadog-agent-rtloader execinfo)
+    add_compile_definitions(HAS_BACKTRACE_LIB)
 endif()
 endif()
 
 ## Backtrace
 if(${CMAKE_SYSTEM_NAME} STREQUAL FreeBSD)
   find_package(Backtrace REQUIRED)
   target_link_libraries(datadog-agent-rtloader ${Backtrace_LIBRARIES})
+  add_compile_definitions(HAS_BACKTRACE_LIB)
 endif()
 
 set_target_properties(datadog-agent-rtloader PROPERTIES

rtloader/rtloader/api.cpp

@@ -12,7 +12,9 @@
 #ifndef _WIN32
 // clang-format off
 // handler stuff
+#ifdef HAS_BACKTRACE_LIB
 #include <execinfo.h>
+#endif
 #include <csignal>
 #include <cstring>
 #include <sys/types.h>
@@ -371,11 +373,14 @@ static inline void core(int sig)
 #    define STACKTRACE_SIZE 500
 void signalHandler(int sig, siginfo_t *, void *)
 {
+#    ifdef HAS_BACKTRACE_LIB
     void *buffer[STACKTRACE_SIZE];
     char **symbols;
 
     size_t nptrs = backtrace(buffer, STACKTRACE_SIZE);
+#    endif
     std::cerr << "HANDLER CAUGHT signal Error: signal " << sig << std::endl;
+#    ifdef HAS_BACKTRACE_LIB
     symbols = backtrace_symbols(buffer, nptrs);
     if (symbols == NULL) {
         std::cerr << "Error getting backtrace symbols" << std::endl;
@@ -387,6 +392,7 @@ void signalHandler(int sig, siginfo_t *, void *)
 
         _free(symbols);
     }
+#    endif
 
     // dump core if so configured
     __sync_synchronize();