posix: add missing HAVE_ALLOCA_H definition, fixes #272

Without this, the build fails with:
```
/var/tmp/portage/net-nntp/nzbget-24.1/work/nzbget-24.1/lib/regex/regex.c: In function ‘set_regs’:
/var/tmp/portage/net-nntp/nzbget-24.1/work/nzbget-24.1/lib/regex/regex.c:7701:39: error: implicit declaration of function ‘alloca’; did you mean ‘calloc’? [-Wimplicit-function-declaration]                        7701 |         prev_idx_match = (regmatch_t*)alloca(nmatch * sizeof(regmatch_t));
      |                                       ^~~~~~
      |                                       calloc
```

cmake/posix.cmake

@@ -8,6 +8,12 @@ option(DISABLE_PARCHECK "Disable parcheck")
 option(USE_OPENSSL "Use OpenSSL" ON)
 option(USE_GNUTLS "Use GnuTLS" OFF)
 
+include(CheckIncludeFile)
+CHECK_INCLUDE_FILE(alloca.h HAVE_ALLOCA_H)
+if(HAVE_ALLOCA_H)
+  add_definitions(-DHAVE_ALLOCA_H)
+endif()
+
 if(NOT DISABLE_TLS AND USE_GNUTLS)
 	set(USE_OPENSSL OFF)
 endif()