It's hard to remember whether it's `-DUSE_HTTPS=mbedTLS` or
`-DUSE_HTTPS=mbedtls`. Even worse for things like `builtin` which we may
have been inconsistent about. Allow for case insensitive options.
When `-DUSE_HTTP_PARSER=...` is specified, ensure that the specified
HTTP Parser is valid, do not fallback to builtin.
Restore `-DUSE_HTTP_PARSER=system` for backcompatibility.
Fix building against system http-parser library by fixing
the find_package() argument. It seems to have been accidentally changed
from HTTPParser to HTTP_Parser in de178d36f, effectively making
the build against system library fail to find it:
```
CMake Warning at cmake/SelectHTTPParser.cmake:3 (find_package):
By not providing "FindHTTP_Parser.cmake" in CMAKE_MODULE_PATH this project
has asked CMake to find a package configuration file provided by
"HTTP_Parser", but CMake did not find one.
Could not find a package configuration file provided by "HTTP_Parser" with
any of the following names:
HTTP_ParserConfig.cmake
http_parser-config.cmake
Add the installation prefix of "HTTP_Parser" to CMAKE_PREFIX_PATH or set
"HTTP_Parser_DIR" to a directory containing one of the above files. If
"HTTP_Parser" provides a separate development package or SDK, be sure it
has been installed.
Call Stack (most recent call first):
src/CMakeLists.txt:97 (include)
CMake Error at cmake/SelectHTTPParser.cmake:11 (message):
http-parser support was requested but not found
Call Stack (most recent call first):
src/CMakeLists.txt:97 (include)
```
Also applies to *_BINARY_DIR.
This effectively reverts 84083dcc8b,
which broke all users of libgit2 that use it as a CMake subdirectory
(via `add_subdirectory()`). This is because CMAKE_SOURCE_DIR refers
to the root-most CMake directory, which in the case of
`add_subdirectory()` is a parent project to libgit2 and thus the paths
don't make any sense to the configuration files. Corollary,
CMAKE_SOURCE_DIR only makes sense if the CMake project is always the
root project - which can rarely be guaranteed.
In all honesty, CMake should deprecate and eventually remove
CMAKE_SOURCE_DIR and CMAKE_BINARY_DIR. It's been the source of headaches
and confusion for years, they're rarely useful over
CMAKE_CURRENT_(SOURCE|BINARY)_DIR or PROJECT_(SOURCE|BINARY)_DIR,
and they cause a lot of confusing configuration and source
code layouts to boot.
Any time they are used, they break `add_subdirectory()` almost 100% of
the time, cause confusing error messages, and hide subtle bugs.
