gh-150311: Fix minor issues in configure.ac for the CYGWIN port

    - Use 'CYGWIN' (uppercase) for ac_sys_system to match the casing used
      in all case-pattern references throughout configure.ac.
    - Fix LDLIBRARY for static builds: use '.a' extension instead of
      '.dll.a' when shared libraries are disabled.
    - Replace hardcoded 'gcc' and 'g++' with '$(CC)' and '$(CXX)' in
      LDSHARED/LDCXXSHARED for Cygwin.

Author: Ar-maan05

configure.ac

@@ -334,7 +334,7 @@ then
 		ac_sys_system=Linux
 		;;
 	*-*-cygwin*)
-		ac_sys_system=Cygwin
+		ac_sys_system=CYGWIN
 		;;
 	*-apple-ios*)
 		ac_sys_system=iOS
@@ -1650,7 +1650,7 @@ else # shared is disabled
   case $ac_sys_system in
     CYGWIN*)
       BLDLIBRARY='$(LIBRARY)'
-      LDLIBRARY='libpython$(LDVERSION).dll.a'
+      LDLIBRARY='libpython$(LDVERSION).a'
       ;;
   esac
 fi
@@ -3694,8 +3694,8 @@ then
 			dnl not implemented yet
 		]);;
 	CYGWIN*)
-		LDSHARED="gcc -shared -Wl,--enable-auto-image-base"
-		LDCXXSHARED="g++ -shared -Wl,--enable-auto-image-base";;
+		LDSHARED="$(CC) -shared -Wl,--enable-auto-image-base"
+		LDCXXSHARED="$(CXX) -shared -Wl,--enable-auto-image-base";;
 	*)	LDSHARED="ld";;
 	esac
 fi