build: Merge pkg-config Libs.private into Libs for static-only builds

A project being built entirely statically will call pkg-config with
--static, which utilises the Libs.private field. Conversely it will
not use --static when not being built entirely statically, even if
there is only a static build of SDL available. This will most likely
cause the build to fail due to underlinking unless we merge the Libs
fields.

This is what the Meson build system does when it generates pkg-config
files. This also also follows the behaviour of sdl2-config.

At the same time, the runtime linker flags are not applicable to
static-only builds so only add them for shared builds.

configure

@@ -647,6 +647,7 @@ ENABLE_STATIC_FALSE
 ENABLE_STATIC_TRUE
 ENABLE_SHARED_FALSE
 ENABLE_SHARED_TRUE
+PKG_CONFIG_LIBS_PRIV
 SDL_RLD_FLAGS
 SDL_STATIC_LIBS
 SDL_LIBS
@@ -25675,7 +25676,7 @@ SDLTEST_DEPENDS=`echo "$SDLTEST_DEPENDS" | sed "s,\\([^ ]*\\)/\\([^ ]*\\)\\.c,\\
 
 # Set runtime shared library paths as needed
 
-if test "x$enable_rpath" = "xyes"; then
+if test "x$enable_rpath" = "xyes" -a "x$enable_shared" = "xyes"; then
   if test $ARCH = bsdi -o $ARCH = freebsd -o $ARCH = linux -o $ARCH = netbsd; then
     SDL_RLD_FLAGS="-Wl,-rpath,\${libdir}"
 
@@ -25723,9 +25724,12 @@ SDL_STATIC_LIBS="$SDL_LIBS $EXTRA_LDFLAGS"
 
 
 if test x$enable_shared = xyes; then
+    PKG_CONFIG_LIBS_PRIV="
+Libs.private:"
     ENABLE_SHARED_TRUE=
     ENABLE_SHARED_FALSE="#"
 else
+    PKG_CONFIG_LIBS_PRIV=
     ENABLE_SHARED_TRUE="#"
     ENABLE_SHARED_FALSE=
 fi