GH-138800: fix variable substitution in python3.pc for Android

$(BLDLIBRARY) substitution works in Makefiles where variables are
expanded at the time of execution. This expansion does not happen in
static files like .pc, so use value of BLDLIBRARY instead of trying to
substitute the value.

Patch stolen from downstream distribution of Python for Termux
(Android): termux/termux-packages#27739

Preferably needs to be backported to both Python 3.13 as well as Python
3.14, along with the main branch

Fixes #138800

Author: thunder-coding

configure.ac

@@ -6424,7 +6424,7 @@ LIBPYTHON=''
 # On Android and Cygwin the shared libraries must be linked with libpython.
 if test "$PY_ENABLE_SHARED" = "1" && ( test -n "$ANDROID_API_LEVEL" || test "$MACHDEP" = "cygwin"); then
   MODULE_DEPS_SHARED="$MODULE_DEPS_SHARED \$(LDLIBRARY)"
-  LIBPYTHON="\$(BLDLIBRARY)"
+  LIBPYTHON="$(BLDLIBRARY)"
 fi
 
 # On iOS the shared libraries must be linked with the Python framework