libcurl.pc: add Cflags.private

Ref: https://manpages.debian.org/bookworm/pkgconf/pc.5.en.html#PROPERTY_KEYWORDS
Ref: https://www.msys2.org/docs/pkgconfig/#cflagsprivate-static-libraries

Closes #14321

CMakeLists.txt

@@ -1877,6 +1877,7 @@ if(NOT CURL_DISABLE_INSTALL)
   if(LIBCURL_PC_REQUIRES_PRIVATE)
     string(REPLACE ";" "," LIBCURL_PC_REQUIRES_PRIVATE "${LIBCURL_PC_REQUIRES_PRIVATE}")
   endif()
+  set(LIBCURL_PC_CFLAGS_PRIVATE "-DCURL_STATICLIB")
 
   # Merge pkg-config private fields into public ones when static-only
   if(BUILD_SHARED_LIBS)
@@ -1888,7 +1889,7 @@ if(NOT CURL_DISABLE_INSTALL)
     set(ENABLE_SHARED         "no")
     set(LIBCURL_PC_REQUIRES   "${LIBCURL_PC_REQUIRES_PRIVATE}")
     set(LIBCURL_NO_SHARED     "${LIBCURL_LIBS}")
-    set(CPPFLAG_CURL_STATICLIB "-DCURL_STATICLIB")
+    set(CPPFLAG_CURL_STATICLIB "${LIBCURL_PC_CFLAGS_PRIVATE}")
   endif()
   if(BUILD_STATIC_LIBS)
     set(ENABLE_STATIC         "yes")

configure.ac

@@ -491,9 +491,12 @@ AM_CONDITIONAL([USE_CPPFLAG_CURL_STATICLIB],
 # targeting a static library and not building its shared counterpart.
 #
 
+LIBCURL_PC_CFLAGS_PRIVATE='-DCURL_STATICLIB'
+AC_SUBST(LIBCURL_PC_CFLAGS_PRIVATE)
+
 CPPFLAG_CURL_STATICLIB=
 if test "x$xc_lt_build_static_only" = 'xyes'; then
-  CPPFLAG_CURL_STATICLIB='-DCURL_STATICLIB'
+  CPPFLAG_CURL_STATICLIB="${LIBCURL_PC_CFLAGS_PRIVATE}"
 fi
 AC_SUBST([CPPFLAG_CURL_STATICLIB])
 

libcurl.pc.in

@@ -38,3 +38,4 @@ Requires.private: @LIBCURL_PC_REQUIRES_PRIVATE@
 Libs: -L${libdir} -lcurl @LIBCURL_NO_SHARED@
 Libs.private: @LIBCURL_LIBS@
 Cflags: -I${includedir} @CPPFLAG_CURL_STATICLIB@
+Cflags.private: @LIBCURL_PC_CFLAGS_PRIVATE@