cmake: fix include ordering issues with bundled deps

When linking against bundled libraries, we include their header
directories by using "-isystem". The reason for that is that we
want to handle our vendored library headers specially, most
importantly to ignore warnings generated by including them. By
using "-isystem", though, we screw up the order of searched
include directories by moving those bundled dependencies towards
the end of the lookup order. Like this, chances are high that any
other specified include directory contains a file that collides
with the actual desired include file.

Fix this by not treating the bundled dependencies' include
directories as system includes. This will move them to the front
of the lookup order and thus cause them to override
system-provided headers. While this may cause the compiler to
generate additional warnings when processing bundled headers,
this is a tradeoff we should make regardless to fix builds on
systems hitting this issue.

(cherry picked from commit ee3d71fb)

src/CMakeLists.txt

@@ -60,7 +60,7 @@ IF(NOT HAVE_REGCOMP_L)
 	CHECK_FUNCTION_EXISTS(regcomp HAVE_REGCOMP)
 	IF(NOT HAVE_REGCOMP)
 		ADD_SUBDIRECTORY("${libgit2_SOURCE_DIR}/deps/regex" "${libgit2_BINARY_DIR}/deps/regex")
-		LIST(APPEND LIBGIT2_SYSTEM_INCLUDES "${libgit2_SOURCE_DIR}/deps/regex")
+		LIST(APPEND LIBGIT2_INCLUDES "${libgit2_SOURCE_DIR}/deps/regex")
 		LIST(APPEND LIBGIT2_OBJECTS $<TARGET_OBJECTS:regex>)
 	ENDIF()
 ENDIF()
@@ -129,7 +129,7 @@ IF (WIN32 AND WINHTTP)
 	IF (MINGW)
 		ADD_SUBDIRECTORY("${libgit2_SOURCE_DIR}/deps/winhttp" "${libgit2_BINARY_DIR}/deps/winhttp")
 		LIST(APPEND LIBGIT2_LIBS winhttp)
-		LIST(APPEND LIBGIT2_SYSTEM_INCLUDES "${libgit2_SOURCE_DIR}/deps/winhttp")
+		LIST(APPEND LIBGIT2_INCLUDES "${libgit2_SOURCE_DIR}/deps/winhttp")
 	ELSE()
 		LIST(APPEND LIBGIT2_LIBS "winhttp")
 		LIST(APPEND LIBGIT2_PC_LIBS "-lwinhttp")
@@ -316,7 +316,7 @@ IF (USE_EXT_HTTP_PARSER AND HTTP_PARSER_FOUND AND HTTP_PARSER_VERSION_MAJOR EQUA
 ELSE()
 	MESSAGE(STATUS "http-parser version 2 was not found or disabled; using bundled 3rd-party sources.")
 	ADD_SUBDIRECTORY("${libgit2_SOURCE_DIR}/deps/http-parser" "${libgit2_BINARY_DIR}/deps/http-parser")
-	LIST(APPEND LIBGIT2_SYSTEM_INCLUDES "${libgit2_SOURCE_DIR}/deps/http-parser")
+	LIST(APPEND LIBGIT2_INCLUDES "${libgit2_SOURCE_DIR}/deps/http-parser")
 	LIST(APPEND LIBGIT2_OBJECTS "$<TARGET_OBJECTS:http-parser>")
 	ADD_FEATURE_INFO(http-parser ON "http-parser support (bundled)")
 ENDIF()
@@ -340,7 +340,7 @@ IF(NOT USE_BUNDLED_ZLIB)
 ENDIF()
 IF(USE_BUNDLED_ZLIB OR NOT ZLIB_FOUND)
 	ADD_SUBDIRECTORY("${libgit2_SOURCE_DIR}/deps/zlib" "${libgit2_BINARY_DIR}/deps/zlib")
-	LIST(APPEND LIBGIT2_SYSTEM_INCLUDES "${libgit2_SOURCE_DIR}/deps/zlib")
+	LIST(APPEND LIBGIT2_INCLUDES "${libgit2_SOURCE_DIR}/deps/zlib")
 	LIST(APPEND LIBGIT2_OBJECTS $<TARGET_OBJECTS:zlib>)
 	ADD_FEATURE_INFO(zlib ON "using bundled zlib")
 ENDIF()