mbedtls: fix inline being used in mbedtls headers

The mbedtls headers make direct use of the `inline` attribute to
instruct the compiler to inline functions. As this function is not C90
compliant, this can cause the compiler to error as soon as any of these
files is included and the `-std=c90` flag is being added.

The mbedtls headers declaring functions as inline always have a prelude
which define `inline` as a macro in case it is not yet defined. Thus, we
can easily replace their define with our own define, which simply copies
the logic of our own `GIT_INLINE` macro.

Author: pks-t

src/streams/mbedtls.c

@@ -26,12 +26,23 @@
 #define GIT_DEFAULT_CERT_LOCATION NULL
 #endif
 
+/* Work around C90-conformance issues */
+#if defined(_MSC_VER)
+# define inline __inline
+#elif defined(__GNUC__)
+# define inline __inline__
+#else
+# define inline
+#endif
+
 #include <mbedtls/config.h>
 #include <mbedtls/ssl.h>
 #include <mbedtls/error.h>
 #include <mbedtls/entropy.h>
 #include <mbedtls/ctr_drbg.h>
 
+#undef inline
+
 mbedtls_ssl_config *git__ssl_conf;
 mbedtls_entropy_context *mbedtls_entropy;