cmake: fix Linux pre-fills for non-glibc (when `_CURL_PREFILL=ON`)

- do not pre-fill `HAVE_LINUX_TCP_H` on Linux.
  `linux/tcp.h` is a Linux kernel userspace header. It's likely
  installed when using glibc and likely missing by default when using
  something else, e.g. MUSL (e.g. on Alpine).

  Therefore always detect it for Linux targets, and only pre-fill it for
  non-Linux ones.

- do not pre-fill `HAVE_GLIBC_STRERROR_R` on Linux.
  To fix it for non-glibc envs, e.g. MUSL (e.g. on Alpine).

Note, the pre-fill option is a disabled by default, internal option and
strongly not recommended outside of curl development.

Closes #19116

diff --git a/CMake/unix-cache.cmake b/CMake/unix-cache.cmake
index 2c85626a49574b2cea9abad9567498a51305c050..a0369e84e52f5d3c8e41001da4bc743443ee424c 100644 (file)
--- a/CMake/unix-cache.cmake
+++ b/CMake/unix-cache.cmake
@@ -142,7 +142,7 @@ set(HAVE_GETRLIMIT 1)
 set(HAVE_GETSOCKNAME 1)
 set(HAVE_GETTIMEOFDAY 1)
 if(CMAKE_SYSTEM_NAME STREQUAL "Linux")
-  set(HAVE_GLIBC_STRERROR_R 1)
+  # Depends on C library.
 else()
   set(HAVE_GLIBC_STRERROR_R 0)
 endif()
@@ -164,7 +164,7 @@ else()
 endif()
 set(HAVE_LIBGEN_H 1)
 if(CMAKE_SYSTEM_NAME STREQUAL "Linux")
-  set(HAVE_LINUX_TCP_H 1)
+  # Requires Linux kernel userspace headers. Expected with glibc. May be missing by default with MUSL.
 else()
   set(HAVE_LINUX_TCP_H 0)
 endif()