Fix detection of warning flags for clang

Using ./configure CC=clang, the following flags are detected
as supported:

checking whether clang supports "-Wgnu89-inline"... yes
checking whether clang supports "-Wno-strict-aliasing"... yes

Which results in a lot of warnings during make:

warning: unknown warning option '-Wunsigned-char' [-Wunknown-warning-option]
warning: unknown warning option '-Wgnu89-inline' [-Wunknown-warning-option]

Clang doesn't support these flags, but the compile check returns a
warning, not an error:

configure:16649: checking whether clang supports "-Wunsigned-char"
configure:16662: clang -E -Wunsigned-char conftest.c
warning: unknown warning option '-Wunsigned-char' [-Wunknown-warning-option]
1 warning generated.
configure:16662: $? = 0
configure:16663: result: yes