configure: Also test for new "check" option

When configure was called without any --enable-... parameters, the HAVE_DRIVERS
define would be undefined, leading to the driver list being empty:

https://sigrok.org/gitweb/?p=libsigrok.git;a=blob;f=configure.ac;h=f846fcdc12171bb487596bf92e398d197a6b74dc;hb=HEAD#l304

https://sigrok.org/gitweb/?p=libsigrok.git;a=blob;f=src/drivers.c;h=6dc2c55cbd18785813186a5e814d0a7d020d1583;hb=HEAD#l50

This was masked when configure is called with --enable-all-drivers as this
parameter is counted as an enabled driver (even though it technically isn't).

Author: abraxa

configure.ac

@@ -301,7 +301,7 @@ m4_define([_SR_DRIVER], [
 	])], [sr_hw_info='no (disabled)'])
 	sr_driver_summary_append "$2" "$sr_hw_info"
 
-	AM_CONDITIONAL([$3], [test "x[$]$3" = xyes])
+	AM_CONDITIONAL([$3], [test "x[$]$3" = xyes || test "x[$]$3" == xcheck])
 	AM_COND_IF([$3], [AC_DEFINE([HAVE_$3], [1], [Whether to support $1 device.]) AC_DEFINE([HAVE_DRIVERS], [1], [Whether at least one driver is enabled.])])
 ])