Add parallel PHP syntax check on template files

Before this, absolutely no automated checks were done on template files
now we at least ensure that they are valid PHP files.

The parallel run should work correctly on macOS and other Unix-like
systems. On Windows, we fallback to sequential execution.

This takes around 1 second on my not so fast 2/4 cores Intel laptop.

Makefile

@@ -8,13 +8,16 @@ entrypoints   = $(wildcard src/*.js)
 
 # Misc variables.
 export PATH             := vendor/bin:node_modules/.bin:$(PATH)
+NPROC                   := 1
 
 ifneq ($(OS),Windows_NT) # Not for Windows
 UNAME_S := $(shell uname -s)
 ifeq ($(UNAME_S),Darwin) # For Mac
 SHELL                   := PATH=$(PATH) /bin/bash
+NPROC                   := $(shell sysctl -n hw.logicalcpu)
 else
 SHELL                   := /bin/bash
+NPROC                   := $(shell nproc)
 endif
 endif
 override GIT_VERSION    := $(shell git --no-pager describe --always --tags)
@@ -173,6 +176,8 @@ lint: lint-php lint-js
 # Lint php code with phpcs.
 .PHONY: lint-php
 lint-php: $(phpcs_dir) vendor
+# Run PHP syntax check on template files in parallel thanks to xargs
+	echo app/view/templates/*.php | xargs -P$(NPROC) -n1 php --syntax-check > /dev/null
 	phpcs --report-full --report-summary --cache=$(phpcs_dir)/result.cache || { printf "run 'make fix'\n\n"; exit 1; }
 
 .PHONY: lint-js