The binary 'mariadb_config' produces its output as some concatenation on a prefix, path and suffix.
The upstream tries to use one binary for all distros, but that does not work as it should for Fedora.

Often producing double prefixes and suffixes to some options.
For example:
#  --cflags        [-I/usr//usr/include/mysql -I/usr//usr/include/mysql/mysql]
  

--- mariadb-connector-c-3.0.4-src/mariadb_config/mariadb_config.c.in	2018-04-24 12:22:12.000000000 +0200
+++ mariadb-connector-c-3.0.4-src/mariadb_config/mariadb_config.c.in_patched	2018-04-26 13:05:49.737644735 +0200
@@ -5,13 +5,13 @@
 
 static char *mariadb_progname;
 
-#define INCLUDE "-I@CMAKE_INSTALL_PREFIX@/@INSTALL_INCLUDEDIR@ -I@CMAKE_INSTALL_PREFIX@/@INSTALL_INCLUDEDIR@/mysql"
-#define LIBS    "-L@CMAKE_INSTALL_PREFIX@/@INSTALL_LIBDIR@/ -lmariadb @extra_dynamic_LDFLAGS@"
+#define INCLUDE "-I@INSTALL_INCLUDEDIR@ -I@INSTALL_INCLUDEDIR@"
+#define LIBS    "-L@INSTALL_LIBDIR@/ -lmariadb @extra_dynamic_LDFLAGS@"
 #define LIBS_SYS "@extra_dynamic_LDFLAGS@"
 #define CFLAGS  INCLUDE
 #define VERSION "@MARIADB_CLIENT_VERSION@"
 #define CC_VERSION "@CPACK_PACKAGE_VERSION@"
-#define PLUGIN_DIR "@CMAKE_INSTALL_PREFIX@/@INSTALL_PLUGINDIR@"
+#define PLUGIN_DIR "@INSTALL_PLUGINDIR@"
 #define SOCKET  "@MARIADB_UNIX_ADDR@"
 #define PORT "@MARIADB_PORT@"
 #define TLS_LIBRARY_VERSION "@TLS_LIBRARY_VERSION@"