lcov: Make JSON module configurable

lcov 1.15 uses core module JSON::PP for parsing gcov-9 output. This
slowed down analysis for my project 10 times (with GCC 5 they uses txt
format).

My changes add an option to configure JSON module for lcov. To reach
past performance I can force lcov to use fast JSON::XS module, e.g.:

$ lcov --capture --directory $products --output-file $file
       --rc lcov_json_module=JSON::XS

As this is not a Perl core module you should install it by yourself:

$ cpan JSON:XS

Regarding performance, here is execution time for a specific file:

JSON::PP (default): 0m10.764s
JSON::XS          : 0m0.542s
Cpanel::JSON::XS  : 0m0.618s
No JSON (GCC 5)   : 0m0.444s

[oberpar: added verbose commit log from pull request, fixed typos,
          renamed rc option to lcov_* to make it more versatile]

Signed-off-by: Odinochenko Aleksey <[email protected]>

Author: tenta4

bin/geninfo

@@ -60,7 +60,6 @@ use Getopt::Long;
 use Digest::MD5 qw(md5_base64);
 use Cwd qw/abs_path/;
 use IO::Uncompress::Gunzip qw(gunzip $GunzipError);
-use JSON::PP qw(decode_json);
 
 if( $^O eq "msys" )
 {
@@ -275,6 +274,7 @@ our $br_coverage = 0;
 our $no_exception_br = 0;
 our $rc_auto_base = 1;
 our $rc_intermediate = "auto";
+our $rc_json_module = "auto";
 our $intermediate;
 our $excl_line = "LCOV_EXCL_LINE";
 our $excl_br_line = "LCOV_EXCL_BR_LINE";
@@ -345,6 +345,7 @@ if ($config || %opt_rc)
 		"geninfo_adjust_src_path"	=> \$rc_adjust_src_path,
 		"geninfo_auto_base"		=> \$rc_auto_base,
 		"geninfo_intermediate"		=> \$rc_intermediate,
+		"lcov_json_module"		=> \$rc_json_module,
 		"geninfo_no_exception_branch"	=> \$no_exception_br,
 		"lcov_function_coverage"	=> \$func_coverage,
 		"lcov_branch_coverage"		=> \$br_coverage,
@@ -493,6 +494,18 @@ if ($rc_intermediate eq "0") {
 	    "'$rc_intermediate'\n");
 }
 
+# Determine JSON module
+my $current_json_module = "JSON::PP";
+if ($rc_json_module ne "auto") {
+	$current_json_module = $rc_json_module
+}
+
+use Module::Load;
+eval "load $current_json_module, 'decode_json'";
+if ($@) {
+	die("Module is not installed: ". "'$current_json_module'\n");
+};
+
 if ($intermediate) {
 	info("Using intermediate gcov format\n");
 	if ($opt_derive_func_data) {