cmake: treat LIBGIT2_PC_REQUIRES as a list

It is indeed a list of dependencies for those which include the static archive.
This is in preparation for adding two possible places where we might add openssl
as a dependency.

src/CMakeLists.txt

@@ -210,7 +210,7 @@ IF(SHA1_BACKEND STREQUAL "OpenSSL")
 	IF(CMAKE_SYSTEM_NAME MATCHES "FreeBSD")
 		LIST(APPEND LIBGIT2_PC_LIBS "-lssl")
 	ELSE()
-		SET(LIBGIT2_PC_REQUIRES "${LIBGIT2_PC_REQUIRES} openssl")
+		LIST(APPEND LIBGIT2_PC_REQUIRES "openssl")
 	ENDIF()
 ELSEIF(SHA1_BACKEND STREQUAL "CollisionDetection")
 	ADD_FEATURE_INFO(SHA ON "using CollisionDetection")
@@ -265,7 +265,7 @@ IF(NOT USE_BUNDLED_ZLIB)
 			LIST(APPEND LIBGIT2_LIBS "z")
 			LIST(APPEND LIBGIT2_PC_LIBS "-lz")
 		ELSE()
-			SET(LIBGIT2_PC_REQUIRES "${LIBGIT2_PC_REQUIRES} zlib")
+			LIST(APPEND LIBGIT2_PC_REQUIRES "zlib")
 		ENDIF()
 		ADD_FEATURE_INFO(zlib ON "using system zlib")
 	ELSE()
@@ -439,6 +439,9 @@ IF (SONAME)
 		SET_TARGET_PROPERTIES(git2 PROPERTIES PREFIX "${LIBGIT2_PREFIX}")
 	ENDIF()
 ENDIF()
+
+LIST(REMOVE_DUPLICATES LIBGIT2_PC_REQUIRES)
+STRING(REPLACE ";" " " LIBGIT2_PC_REQUIRES "${LIBGIT2_PC_REQUIRES}")
 STRING(REPLACE ";" " " LIBGIT2_PC_LIBS "${LIBGIT2_PC_LIBS}")
 CONFIGURE_FILE(${libgit2_SOURCE_DIR}/libgit2.pc.in ${libgit2_BINARY_DIR}/libgit2.pc @ONLY)