Further improve SQLite support for CMake users.

Unfortunately previous commit was only a partial fix, because it broke
SQLite support on platforms w/o pkg-config, e.g. Windows. To be honest
I just forgot about messy Windows.

Now if there is no pkg-config, then user must provide two variables:
SQLITE3_INCLUDE_DIRS and SQLITE3_LIBRARIES if (s)he wants to use SQLite
backend. These variables are added to cmake-gui for her/his convenience
unless they are set by FindPkgConfig module.

pkg-config should work also now in Cygwin.

CMakeLists.txt

@@ -33,12 +33,23 @@ ELSEIF ()
     SET(SHA1_TYPE "builtin" CACHE STRING "Which SHA1 implementation to use: builtin, ppc")
 ENDIF ()
 
-# Try to find SQLite3 to compile the SQLite backend
-IF (NOT WIN32)
-	INCLUDE(FindPkgConfig)
+INCLUDE(FindPkgConfig)
+
+# Show SQLite3 settings in GUI (if they won't be found out)
+SET(SQLITE3_INCLUDE_DIRS "" CACHE PATH "SQLite include directory")
+SET(SQLITE3_LIBRARIES "" CACHE FILEPATH "SQLite library")
+
+# Are SQLite3 variables already set up? (poor Windows/no pkg-config/no sqlite3.pc)
+IF (SQLITE3_INCLUDE_DIRS AND SQLITE3_LIBRARIES)
+	SET(SQLITE3_FOUND 1)
+ENDIF ()
+
+# Try to find SQLite3 via pkg-config
+IF (PKG_CONFIG_FOUND AND NOT SQLITE3_FOUND)
 	pkg_check_modules(SQLITE3 sqlite3)
 ENDIF ()
 
+# Compile SQLite backend if SQLite3 is available
 IF (SQLITE3_FOUND)
 	ADD_DEFINITIONS(-DGIT2_SQLITE_BACKEND)
 	INCLUDE_DIRECTORIES(${SQLITE3_INCLUDE_DIRS})