Fix libcares.pc generation for static MingW* cross builds

configure.ac

@@ -130,6 +130,7 @@ AC_PROG_LIBTOOL
 AC_MSG_CHECKING([if we need CARES_BUILDING_LIBRARY])
 use_cppflag_cares_building_library="no"
 use_cppflag_cares_staticlib="no"
+CPPFLAG_CARES_STATICLIB=""
 case $host in
   *-*-mingw*)
     AC_MSG_RESULT(yes)
@@ -139,6 +140,7 @@ case $host in
     then
       AC_MSG_RESULT(yes)
       use_cppflag_cares_staticlib="yes"
+      CPPFLAG_CARES_STATICLIB="-DCARES_STATICLIB"
     else
       AC_MSG_RESULT(no)
     fi
@@ -149,6 +151,7 @@ case $host in
 esac
 AM_CONDITIONAL(USE_CPPFLAG_CARES_BUILDING_LIBRARY, test x$use_cppflag_cares_building_library = xyes)
 AM_CONDITIONAL(USE_CPPFLAG_CARES_STATICLIB, test x$use_cppflag_cares_staticlib = xyes)
+AC_SUBST(CPPFLAG_CARES_STATICLIB)
 
 dnl **********************************************************************
 dnl platform/compiler/architecture specific checks/flags

libcares.pc.in

@@ -15,6 +15,6 @@ Description: asynchronous DNS lookup library
 Version: @VERSION@
 Requires: 
 Requires.private: 
-Cflags: -I${includedir}
+Cflags: -I${includedir} @CPPFLAG_CARES_STATICLIB@
 Libs: -L${libdir} -lcares
 Libs.private: @CARES_PRIVATE_LIBS@