cmake: move regex build instructions into subdirectory

Extract code required to build the regex library into its own
CMakeLists.txt, which is included as required.

CMakeLists.txt

@@ -453,8 +453,9 @@ ENDIF()
 
 # Include POSIX regex when it is required
 IF(WIN32 OR AMIGA OR CMAKE_SYSTEM_NAME MATCHES "(Solaris|SunOS)")
-	LIST(APPEND LBIGIT2_INCLUDES "${CMAKE_SOURCE_DIR}/deps/regex")
-	SET(SRC_REGEX "${CMAKE_SOURCE_DIR}/deps/regex/regex.c")
+	ADD_SUBDIRECTORY("${CMAKE_SOURCE_DIR}/deps/regex" "${CMAKE_BINARY_DIR}/deps/regex")
+	LIST(APPEND LIBGIT2_INCLUDES "${CMAKE_SOURCE_DIR}/deps/regex")
+	LIST(APPEND LIBGIT2_LIBS regex)
 ENDIF()
 
 # Optional external dependency: http-parser
@@ -599,7 +600,7 @@ ENDIF()
 
 CONFIGURE_FILE(src/features.h.in git2/sys/features.h)
 
-SET(GIT2INTERNAL_OBJECTS ${SRC_H} ${SRC_GIT2} ${SRC_OS} ${SRC_ZLIB} ${SRC_HTTP} ${SRC_REGEX} ${SRC_SSH} ${SRC_SHA1})
+SET(GIT2INTERNAL_OBJECTS ${SRC_H} ${SRC_GIT2} ${SRC_OS} ${SRC_ZLIB} ${SRC_HTTP} ${SRC_SSH} ${SRC_SHA1})
 
 LIST(APPEND LIBGIT2_INCLUDES ${CMAKE_CURRENT_BINARY_DIR} "${CMAKE_SOURCE_DIR}/src" "${CMAKE_SOURCE_DIR}/include")
 

deps/regex/CMakeLists.txt

+INCLUDE_DIRECTORIES(".")
+ADD_LIBRARY(regex STATIC "regex.c" "regex.h")