Convert all make calls to POSIX format.

Suggested by @thesamesam
Link: https://github.com/gentoo/gentoo/pull/42447#discussion_r2127539884

--- a/Makefile
+++ b/Makefile
@@ -5,9 +5,9 @@
 PROGRAM = analog
 
 $(PROGRAM): ALWAYS
-	cd src && make
+	$(MAKE) -C src
 
 clean:
-	cd src && make clean
+	$(MAKE) -C src clean
 
 ALWAYS:
--- a/src/Makefile
+++ b/src/Makefile
@@ -98,13 +98,13 @@
 # There doesn't seem to be a good way to write all these rules in a generic
 # form that works for all "make" programs
 bzip2: ALWAYS
-	cd bzip2 && $(MAKE) 'CC=$(CC)' 'ALLCFLAGS=$(ALLCFLAGS)'
+	$(MAKE) -C bzip2 'CC=$(CC)' 'ALLCFLAGS=$(ALLCFLAGS)'
 
 libgd: ALWAYS
-	cd libgd && $(MAKE) 'CC=$(CC)' 'ALLCFLAGS=$(ALLCFLAGS)'
+	$(MAKE) -C libgd 'CC=$(CC)' 'ALLCFLAGS=$(ALLCFLAGS)'
 
 libpng: ALWAYS
-	cd libpng && $(MAKE) 'CC=$(CC)' 'ALLCFLAGS=$(ALLCFLAGS)'
+	$(MAKE) -C libpng 'CC=$(CC)' 'ALLCFLAGS=$(ALLCFLAGS)'
 
 pcre2: ALWAYS
 	chmod 755 pcre2/132html pcre2/ar-lib pcre2/CheckMan pcre2/CleanTxt pcre2/compile pcre2/config.guess pcre2/config.sub pcre2/configure pcre2/depcomp pcre2/Detrail pcre2/install-sh pcre2/missing pcre2/perltest.sh pcre2/PrepareRelease pcre2/RunGrepTest pcre2/RunTest pcre2/test-driver
@@ -113,10 +113,10 @@
 	cd pcre2 && ./configure && $(MAKE) 'CC=$(CC)' 'ALLCFLAGS=$(ALLCFLAGS)'
 
 unzip: ALWAYS
-	cd unzip && $(MAKE) 'CC=$(CC)' 'ALLCFLAGS=$(ALLCFLAGS)'
+	$(MAKE) -C unzip 'CC=$(CC)' 'ALLCFLAGS=$(ALLCFLAGS)'
 
 zlib: ALWAYS
-	cd zlib && $(MAKE) 'CC=$(CC)' 'ALLCFLAGS=$(ALLCFLAGS)'
+	$(MAKE) -C zlib 'CC=$(CC)' 'ALLCFLAGS=$(ALLCFLAGS)'
 
 alias.o: alias.c $(HEADERS) Makefile
 	$(CC) $(ALLCFLAGS) -c alias.c