Use strtod_l() to correctly parse decimal numbers in French & other locales

Test with, eg.:
$ LC_ALL=fr_FR.utf-8 ./hb-view NotoSansArabic-VF.ttf بهداد --variations wght=1.2

configure.ac

diff --git a/configure.ac b/configure.ac
index 9151abc..d65cae8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -69,8 +69,8 @@
 ])
 
 # Functions and headers
-AC_CHECK_FUNCS(atexit mprotect sysconf getpagesize mmap isatty)
-AC_CHECK_HEADERS(unistd.h sys/mman.h)
+AC_CHECK_FUNCS(atexit mprotect sysconf getpagesize mmap isatty newlocale strtod_l)
+AC_CHECK_HEADERS(unistd.h sys/mman.h xlocale.h)
 
 # Compiler flags
 AC_CANONICAL_HOST