Treat -isystem and -idirafter as -I Cflags since they control the
compiler search path. Adjust the sysroot handling so that the arguments
to these options have the sysroot prefixed. However, leave them out of the
system Cflags handling since these directives are explicitly trying to
adjust the compiler's system header search behavior.
The special-flags test case output needs adjustment since all the flags
are now considered -I flags and come out in the order specified in the
pc file.
https://bugs.freedesktop.org/show_bug.cgi?id=97337
MSYS mangles UNIX paths on execution such that it prepends its root in
Windows format. This affects PKG_CONFIG_SYSROOT_DIR and thus the
check-sysroot test. Detect MSYS from the OSTYPE environment variable and
adjust the expected test results to match. This likely means that
sysroot support doesn't actually work on MSYS, but there probably aren't
any toolchains that support sysroot there, anyway.
Often the expected results for the indirect dependency tests fell behind
because it's not a typical test scenario. However, since the results are
always the same as --static, they can just use the same results and the
test can be run conditionally without --static based on configuration.
